The CHIEF JUSTICE

delivered the opinion of the court.

We are met at the outset of this case with a question of jurisdiction. All the parties, plaintiff as well as defendant, are citizens of the State of New York. The power of the Circuit Court, therefore, to entertain the cause, if it exists at all, must be found in the jurisdiction conferred by the patent laws.

The suit is in equity against a patentee by one who claims to be his assignee, to restrain him from infringing upon rights under his patent, which are alleged to have been assigned. The Circuit Court has jurisdiction of all suits arising under the patent laws, and has power, upon a bill in equity filed by a party aggrieved, to grant injunctions, according to the course and principles of courts of equity, to *219 prevent the violation of any right secured by patent. Every patent, or any interest therein, is by statute made assignable by an instrument in writing, and the patentee or his assignee may, in like manner, grant and convey an exclusive, right under his patent throughout any specified part of the United States. All such assignments must be recorded in the Patent Office within three months from the time of their execution. This power of assignment has been so construed by the courts as to confine it to the transfer of an entire patent, an undivided part thereof, or the entire interest of the patentee or undivided part thereof within and throughout a certain specified portion of the United States. One holding such an assignment is an assignee within the meaning of the statute, and may prosecute in the Circuit Court any action that may be necessary for the protection of his rights under the patent.

The title of the complainant in this case grows out of what is termed in the answers “ a grant and supplementary agreement,” executed in “ two parts,” between Littlefield, the patentee, and Treadwell & Perry. The “grant” is one of the parts, and the “ supplementary agreement” the other. Tlie grant, taken by itself, contains, in most unmistakable language, an absolute conveyance by the patentee of his patent and inventions described, and all improvements thereon, within and throughout the States of New York and Connecticut, and an agreement by the assignees to pay a royalty on all patented articles sold, with a clause of forfeiture in case of non-payment or neglect, after due notice, to make and sell the patented articles to the extent of a reasonable demand therefor. This grant was duly recorded in the Patent Office six days after its execution.

The supplementary agreement was never recorded. It contained, among other things, a stipulation to the effect that nothing in the assignment should give to Treadwell & Perry the right to use or apply the principle of the patent to furnaces erected in cellars or basements of houses for the purpose of heating several rooms, it being the intention of the patentee to reserve that to himself. It also contained *220 certain other stipulations between the parties intended for the protection of their respective rights and the regulation of their conduct under the assignment. The defendants now contend that by reason of this reservation, and these several stipulations, the title of Treadwell & Perry, under the grant, has been reduced from that of assignees to mere licensees.

Undoubtedly, for the purpose of ascertaining the intention of the parties in making their contract, the two instruments, executed as they were at the same time, and each referring to the other, are to be construed together. If, when so construed, they shall be found to convey to the assignees the title to the patent and inventions and grant a license back from the assignees to the patentee of the right to use the patent and its principle in the manufacture of the designated furnaces, the Circuit Court had jurisdiction of the cause.

When the “grant” was placed on record, Treadwell & Perry became the apparent owners of the entire patent and inventions throughout the specified territory. Neither the agreement to account and pay the royalty nor the clause of forfeiture for non-performance contained in that instrument reduced them to the positiou of licensees. The agreement to account and pay formed part of the consideration of the assignment, and was in effect an agreement to pay at a future time a sum to be determined by the number of articles made and sold. For the non-payment or other non-performance a forfeiture might be enforced as for condition broken, but until it was enforced the title granted remained in the assignees.

The supplementary agreement contained a provision that Littlefield should sue infringers “ in his own name or otherwise,” and also defend all suits against Treadwell & Perry for alleged infringements of other patents by the use of his, and this it is alleged is evidence of the intention of the parties to make the grant effective ouly as a license. It needs only a slight examination of that clause in the contract, however, to become satisfied that it was intended only as a provision for placing on Littlefield the costs and expenses *221 of all such litigation, as well as all damages for infringements growing out of the use of the inventions by the assignees. The suits were to be prosecuted in his name-, or otherwise, as circumstances should require, and he was to be at all the costs and expense of maintaining his patents. That is the extent of the provision.

Upon the argument, the reservation of the right to use the principle of the patent, and inventions in the manufacture of furnaces seemed to be relied upon with more confidence as establishing this claim on the part of the defendants. All agree that the intention of the parties, when ascertained by. an examination of both the instruments, must govern in this action where only the parties themselves are interested. There are no intervening innocent third persons. Jagger, the partner of Littlefield, who is codefendant with him, is charged with full notice of the rights of Treadwell & Perry, and others claiming under them.

It is a significant fact that the agreement'was executed in two parts. Ordinarily the whole of such a contract is embodied in a single instrument. Another important fact is, that only one of the parts is recorded, and that the one which, taken by itself, places the title in Treadwell & Perry. The record is intended for the benefit of the public. Bond fide purchasers look to it for their protection. The record of the grant alone, therefore, furnishes the strongest evidence of the intention of the. parties to give effect to the two instruments as an assignment. It is true that in the recorded part reference is made to the other, but the manner of the reference is not such as to indicate that the unrecorded part contained anything to defeat the title granted by that which was recorded. The language is, “in consideration of one dollar, and of the agreements herein contained on the part of the parties of the second part, and of the agreements contained in a certain agreement this day executed betweeu the parties hereto, and bearing even date herewith, hath, and by these presents doth, assign,” &e. And again : “ It is expressly understood and agreed betweeu the said parties that in case said party of the first part shall *222 well aud faithfully keep aud perform all the agreements herein, and in the aforesaid agreement bearing even date herewith contained, and said parties of the second part shall, &e., neglect, &c., that this assignment aud transfer shall thereafter be void and of no effect,” &e. This is undoubtedly sufficient to charge purchasers with notice of the execution of the supplementary agreement, and possibly of its provisions, but it falls far short of indicating an' intention of-the parties, by anything contained in the unrecorded instrument, to limit or defeat the assignment made in consideration of it. The most that can be inferred from such lan-. guage is, that the parties had stipulated between themselves, not as to the legal effect of the recorded instrument, but as to their obligations or equitable rights under it. We think, therefore, that Treadwell & Perry were the assignees of Littlefield within the meaning of the patent laws, and that they and those claiming under them may sue in the Circuit Courts to prevent an infringement upon their rights.

But even if they are not technically assignees, we think this action is, nevertheless, maintainable. They certainly had the exclusive right to the use of the patent for certain purposes within their territory. They thus held a right under the patent. The claim is that this right has been infringed. To determine the suit, therefore, it is necessary to inquire whether there has been an infringement, and that involves a construction of the patents. The act of Congress provides that all actions, suits, controversies, and cases arising under any law of the United States granting or confirming to inventors the exclusive right to their inventions or discoveries shall be originally cognizable, as well in equity as at law, in the Circuit Courts,” &c. An action which raises a question of infringement is an action arising “ nuder the law,” and one who has the right to sue for the infringement may sue in the Circuit Court. Such a suit may involve the construction of a contract as well as the patent, but that will not oust the court of its jurisdiction. If the patent is involved it carries with it the whole case.

*223 A mere licensee cannot sue strangers who infringe. In such case redress is obtained through or in the name of the patentee or his assignee. Here, however, the patentee is the infringer, and as he cannot sue himself, the licensee is powerless, so far as the courts of the United States are concerned, unless he can sue in his own name. A court of equity looks to substance rather than form. When it has jurisdiction of parties it grants the appropriate relief without regard to whether they come as plaintiff or defendant. In this case the person who should have protected the plaintiff against all infringements has become himself the infringer. He held the legal title to his pateut in trust for his licensees. He has been faithless to his trust, and courts of equity are always open for the redress of such a wrong. This wrong is an infringément. Its redress involves a suit, therefore, arising under the patent laws, and of that suit the Circuit Court has jurisdiction..

It is next asserted that the complainant has. not by his proof shown himself to be the assignee of Treadwell & Perry. They, on the 25th of March, 1862, assigned all their interest to George W. Sterling. He became dissatisfied with his purchase, and, by agreement of parties, the sale was can-celled, he giving effect to the cancellation by executing a reassignment to Treadwell & Perry, bearing date June 2d, 1862. Under date of April 7th, 1862, Treadwell & Perry executed another assignment to one. Dickey. Both the reassignment from Sterling and the assignment to Dickey were left at the Patent Office for record on the 26th June, 1862, and on the 2d July Dickey assigned to Mary J. Perry, in whose name the suit was commenced.

It is now claimed that this proof shows title in Treadwell & Perry, inasmuch as Sterling reassigned to them after they had assigned to Dickey. Mrs. Perry was the wife of John S. Perry, one of the firm, and he is now a party to the suit, having upon her death succeeded to all her rights, as trustee under her will. Tread well, the other member of the firm, has been several times in the progress of the cause examined *224 as a witness, and has testified that Dickey became the owner of the patents under a transfer to which be consented. It is clear, therefore, that Mrs. Perry at the commencement of the action was in equity, if not in law, the owner of whatever had been assigned by Littlefield, and that if Treadwell & Perry bad the legal title, they held it in trust for her, and will be estopped by a decree in her favor from setting up as against Littlefield any beneficial interest under it. At an earlier stage of the proceedings it might have been proper to make. Treadwell a party, but upon the case as it now stands no possible harm can result to the defendants from a decree against them in his absence.

This brings us to a consideration of the merits of the case.

On the 15th April, 1851, a patent was issued to Littlefield for a certain improvement in cooking-stoves, and on the 30th December, 1852, he filed in the Patent Office his application for another improvement in stoves, devised “ for the purpose of economizing and burningthe gases generated by the combustion of anthracite coals.” On the 5th April, 1853, he executed the grant and supplementary agreement already referred to. In the grant, after reciting that he held a patent bearing date April 15th, 1851, “ for a coal-burner so constructed as to produce combustion of the inflammable gases of anthracite coals,” and that he had “ made application to the Patent Office at Washington for letters-patent securing to him a certain improvement in the invention so as aforesaid patented to him,” and that such application was then pending,-he proceeded to assign all the right, title, and interest .which he then had, or might thereafter have, “ in or to the aforesaid inventions, improvement, and patent, or the patent or patents that may be granted for said inventions or any improvement therein, and on any extension or extensions thereof within and throughout the district, &c., for and during the term for which the aforesaid letters-patent wore granted, and the terms for which- any patent for the aforesaid improvement or any improvement or improvements thereof may be granted,” &c. The application of *225 December 80th, 1852, was rejected at the Patent Office, and finally withdrawn by Littlefield on the 22d day of July, 1853, he at the same time filing another application for “ a new and useful improvement iii stoves,” so devised as “to burn the gaseous or more inflammable elements of the coal in contact with its more refractory portions, and thus secure a complete combustion of them both.” Upon this application a patent was issued January 20th, 1854. All the patents outstanding, and the subject of this controversy, are admitted to be reissues of this or improvements upon it. Little-field and his codefendant do not deny that they have used the patents issued after January, 1854, and if the title to them passed under the assignment of April, 1853, it is admitted that such use is an infringement and that the complainant is entitled to a decree. The simple question, then, presented for our consideration is as to the effect to be given to this assignment.

It is well settled that a recorded assignment of a perfected invention, made before a patent has issued, carries with it the patent when issued, * and that reissues are not patents for new inventions, but amendments of old patents. If a reissue is obtained with the consent of an assignee, it inures at once to his benefit; if without, he has his election to accept or reject it.

The. parties have themselves agreed that the invention of 1852 is an improvement upon the patent of 1851. In the grant the patent is described as being “ for a coal-burner, so constructed as to produce combustion of the inflammable gases of-anthracite coal,” and the application as being for an improvement upon the patent. It is true that the application is not referred to by its date, but there can be no doubt as to its identity, because the language adopted to describe the patent is not that of the claim in the patent itself, but of the application of 1852. Besides, the application is said to be then pending, and it is not pretended that Littlefield had any other on file in the Patent Office at that *226 date. This relieves us from an examination of the specifications in the patent and application, for the purpose of ascertaining whether in point of fact the one was an improvement upon the other. Littlefield having agreed that it was, and having induced Treadwell & Perry to purchase by reason of this agreement, cannot now deny it.

It is clear, also, that the idea which Littlefield had in mind, and which he was endeavoring by his devices to make practically useful, was greater economy in the use of the inflammable gases of coal to produce combustion. It is not important in this suit that the patent, which had then been obtained, was not in fact suited for that purpose. It is sufficient that it was intended to be so. The subsequent devices, better adapted to the end to be -accomplished, may therefore properly be regarded as improvements upon the original invention. They produce a stove doing the same thing which the first was intended to do, but doing it better. This is the proper office of an improvement.

The assignment in this case, by its express terms, covers all improvements in the original patent or the invention described in the application of 1852. It carried with it the legal title to the existing patent. If one had been issued upon the application, that, too, would have inured to the benefit of the assignee, because in that case it would have been the assignment of a perfected invention. Without considering whether the invention upon which the patent of 1854 issued was not, in fact, the same to all intents and purposes as that of 1852, it is sufficient for the purposes of this case that it was an improvement upon it, or perhaps more properly, that invention perfected. An assignment of an imperfect invention, with all improvements upon it that the inventor may make, is equivalent in equity to an assignment of the perfected results. The assignment in this case being such a one, the assignees became in equity the owners of the patent granted upon the perfected invention; that is to say, of the patent of 1854. Littlefield took the legal title in trust for them, and should convey. Courts of equity in proper cases consider that as done which should be. If *227 there exists an obligation to convey at. once, snch courts will oftentimes proceed as if it had actually been made.

There is here no attempt to obtain the specific performance of a contract, but to restrain this patentee from infringing upon rights which, in a court of equity, he is deemed to have assigned. In other words, this complainant is in equity an assignee, and entitled to protection as such. If the assignment in precisely its present form had been executed after the last reissue was granted, we think it would hardly be claimed that the legal title to all the present outstanding patents did not pass with it. What such an assignment could do in respect to legal titles this has done in respect to such as are equitable. The contest is now between an assignor in equity and his assignee. A court of equity will in such a case give the same effect to an equitable title that it would to one that was legal.

It is next contended that the assignment in this case was forfeited before the commencement of this action, because of the failure of Treadwell & Perry to perform its conditions. There is no proof that the royalty on the stoves made and sold before the action was commenced was sufficient to discharge that part of the debt due from Littlefield to Tread-well & Perry, which was first to be paid out of it before anything was payable to him, and there could be no forfeiture for a neglect to make and sell, until after reasonable notice of the default. No such notice is proven or even claimed.

It is next insisted that if the plaintiff claims the benefit of the last reissues, he puts it out of his power to have damages for infringements previous to their date. The original bill in this cause was filed August 27th, 1862. Everything since that time has been done pendente lite. The first reissue was granted November 19th, 1861, and the first patent for an improvement on the patent of 1854 was issued on the 27th June previous. All in the way of reissues or improvements except these has been done pending the suit. The litigation gathers to its harvest the fruits of the labors of Littlefield and his associates during its pendency. His infringement *228 and that of his codefendant Jagger, claiming under him, commenced in 1862, only a short time before the action was commenced. The question presented by this objection is, therefore, comparatively unimportant; but if it were not, the result would be the same. For as Littlefield held his patents all the time in trust for these assignees to the extent of the territory they owned, he must account to them for the profits he has made by the unlawful use of the trust property.

We are, therefore, clearly of the opinion that the complainant is in equity the assignee of Littlefield, and that he is entitled to recover of the defendants the profits they have made out of these infringements upon his rights. So far there is no error in the court below.

We nowT come to the decree itself. The plaintiff is entitled, as has been seen, to recover of the defendants the profits they have made from the use of the several inventions within the assigned territory; but the decree directed an account of “ all the profits, gains, and advantages which the said defendants, or either of them, have received, or which have arisen or accrued to them, or either of them, from the manufacture, use, or sale of stoves within the States of New York and Connecticut, embracing the improvements described in and covered by the said letters-patent, and the reissues thereof, or any of them.” An account stated upon these principles has been approved by the court in the decree appealed from.

The decree is, as we think, too broad. After the interlocutory decree below settling the principle of the accounting, the case of Mowry v. Whitney * was decided in this court. It was there held that the question to be determined in such a case as this was, “what advantage did the defendant derive from using the complainant’s invention over what he had in using other processes then open to the public, and adequate to enable him to obtain an equally beneficial re- *229 suit? The fruits of that advantage are his profits.” For such profits he is compelled to account as damages.

Here the order is to account for all profits received from, the manufacture, &c., of stoves, émbraeing the improvements covered by any of the patents. This would cover all the. profits made upon a stove having in it any one of the improvements patented. The true inquiry is as to the profits which the defendants have realized as the consequence of the improper use of these improvements. Such profits belong to the plaintiff, and should be accounted for to him. The account of the master may not charge the defendants with more than the complainant is entitled to recover. The conduct of the defendants in withholding statements which it would seem they ought to be able to make, and their evident unwillingness to account, would induce us to sustain the report had the order of reference been less broad. As it is, we think the decree, so far as it settles the principles of the accounting for profits, must be reversed, and that the inquiry before the master must be confined to an account of the profits received by the defendants as the direct result of the use within the assigned territory of the several inventions involved in the case.

This reverses the decree.

Many exceptions were taken to the master’s report. Some .were as to the matters of form, and others were directed to the principles of the accounting as settled by the decree. It is unnecessary to consider these further. Another account may dispose of them all.

The Circuit Court, however, in rendering its final decree, added interest to the amount found by the master to be due upon the account for profits. In Mowry v. Whitney it was held that interest is hot allowable in such cases, except under peculiar circumstances. The testimony thus far presented in this case does not, in our opinion, justify such an allowance. It will be for the court to determine, upon the coming in of the new report, accompanied by other evidence, whether the conduct of the. defendants has been such a3 to subject *230 them to liability in this particular. Profits actually realized are usually, in a ease like this, the measure of unliquidated damages. Circumstances may, however, arise which would justify the addition of interest in order to give complete indemnity for losses sustained by wilful infringements.

Decree reversed to the extent hereinbefore indicated, and the cause remanded, with instructions to take a new account of profits and proceed

In accordance with this opinion.

Gavler v. Wilder, 10 Howard, 477.

14 Wallace, 620.