The defendants appeal from a judgment entered on a jury’s special verdicts. See Mass.R.Civ.P. 49(a), 365 Mass. 812 (1974).2 Specifically, they urge, that the court erred (1) in charging the jury on the issue of the defendants’ negligence because that issue had not been properly raised by the plaintiffs complaint; (2) that the evidence was insufficient to support a finding of negligence; (3) that a photocopy of an undated letter was improperly admitted in evidence; and (4) that a master’s report was improperly (untimely) filed.
On the basis of the record, the jury could have found the following facts: In January, 1975, the defendant Mel-pet Stables, Inc. (Melpet), which operated a stable for the sale, exchange, schooling, boarding and letting of horses for riding, hired the plaintiff as a laborer. The defendant Melanson was the treasurer and manager of Melpet. In May, Melanson directed the plaintiff to pick up the horse of one Coffin for boarding. Sometime after the Coffin horse was boarded at Melpet, the plaintiff saw it rear *327while Coffin’s daughter was riding it. The plaintiff reported the incident to Melanson, who suggested that the plaintiff tell Coffin about it. Coffin agreed with the plaintiff to pay for the necessary "schooling” of the horse. The plaintiff, in undertaking to school the horse, acted under an independent agreement with Coffin. At the time of the plaintiff’s injury, Melanson, who had been training horses for more than forty years, was present and directed the lesson. Under his direction, one Bishop, another employee, worked with the horse. The horse proved a recalcitrant pupil and threw Bishop three times. Melanson directed the plaintiff into the saddle and instructed him in what to do when he got there. The plaintiffs stint as equine teacher came to an abrupt and unhappy end when the horse reared and fell on him. That mishap resulted in back injuries to the plaintiff.
The plaintiff brought an action against both Melanson and Melpet, alleging that they were negligent (the latter through its agent, Melanson) in the supervision and direction of the schooling of the horse, and that he was injured as a consequence of their negligence.3 The case was referred to a master; hearings began April 22, 1976, and were concluded July 15,1976. In his final report the master found that the plaintiff was employed by the defendants, that he was injured in the course of his employment, that the defendants failed to carry workmen’s compensation insurance, and that the defendants were negli*328gent. The defendants’ motions to strike the master’s report were denied. After the jury trial, the defendants’ motions for judgment notwithstanding the verdict and for a new trial on the negligence issue were also denied.
1. The defendants urge that they were surprised by the fact that the judge instructed the jury on the issue of negligence, having expected, they say, only a charge on the employer-employee relationship. In view of the allegations of negligence in the plaintiff’s complaint, particularly directed to Melanson’s directions of the schooling of the horse, it is difficult to credit the defendants’ argument. In any event, the defendants failed to object to the fact that the judge had instructed the jury on the alternate negligence theory, and so the issue may not now be raised on appeal. An appeal based on erroneous instructions can be made only if objections to the instructions were raised at trial. Ses Mass.R.Civ.P. 51(b), 365 Mass. 816 (1974). See also Narkin v. Springfield, 5 Mass. Ct. 489 (1977); Worcester v. Eisenbeiser, 7 Mass. App. Ct. 345, 349 (1979).
2. As to the defendants’ assertion that the record lacks evidence of negligence on the part of the defendants, our examination of the record discloses ample support for the jury’s finding of negligence. The master had made a finding of negligent direction of the horse’s tutorial by Melanson. Under Mass.R.Civ.P. 53(e)(3), 365 Mass. 820 (1974), the findings of a master, upon their introduction in evidence, establish a prima facie case as to the facts found by him. "[PJrima facie evidence is ’evidence,’ remains evidence throughout the trial, and is entitled to be weighed like any other evidence upon any question of fact to which it is relevant.” Cook v. Farm Serv. Stores, Inc., 301 Mass. 564, 566 (1938).
Although prima facie evidence ’’may be met and overcome by evidence sufficient to warrant a contrary conclusion” Anderson’s Case, 373 Mass. 813, 817 (1977), the defendants’ evidence was sufficient, at best, only to overcome the compelling effect of the master’s findings and to *329leave the question of negligence to the jury on all the evidence, including the master’s report. There was enough in that evidence, and in the report, to warrant the jury’s conclusion that the defendant was negligent. See New England Acceptance Corp. v. American Manufacturers Mut. Ins. Co., 4 Mass. App. Ct. 172,177 (1976), S.C. 373 Mass. 594 (1977).
But if, the defendants argue, the plaintiff was acting under an independent contract with the owner of the horse at the time of his injury (as the jury found), then Melanson’s instructions constituted a "gratuitous undertaking” for the plaintiffs benefit. Under such circumstances, liability could not be imposed on either defendant, if Melanson’s conduct constituted only ordinary negligence; it would have to amount to gross negligence or wilful, wanton or reckless conduct. Massaletti v. Fitzroy, 228 Mass. 487, 508 (1917). Both sides agree, however, that liability for ordinary negligence can be imposed if the jury could have found that the defendants derived some business advantage from Melanson’s instructions in the schooling of the horse. Beaulieu v. Lincoln Rides, Inc., 328 Mass. 427, 428-429 (1952). Magro v. Byington, 344 Mass. 255, 257 (1962). Melanson/Melpet’s business interest in schooling Coffin’s horse is not indiscernible. Mel-pet’s agreement with Coffin to board and care for his horse was jeopardized by the erratic behavior of the animal; by permitting, or causing, the plaintiff to school the horse, Melpet accommodated a service to Coffin which was likely to keep the horse at the Melpet stables.4
The jury could reasonably find the defendants liable for ordinary negligence on the evidence. They had the master’s findings. They heard testimony that Melanson operated the stables, had vast experience with horses, and had agreed to board the Coffin horse. Melanson was the plain*330tiff’s boss. The plaintiff could not have schooled the Coffin horse without Melanson’s permission. Melanson’s presence and active involvement in supervising the schooling of the horse were designed to maintain a business advantage. The finding that the plaintiff acted under an independent contract at the time of his injury does not require a different result. Cf. Restatement (Second) of Torts § 410 & Comment c (1965).
3. The master’s report was timely filed, and hence was properly introduced in evidence at trial. Under Mass.R.Civ.P. 53(e) (1), as amended, 367 Mass. 917 (1975), the master has forty-five days from the close of evidence to file the report, unless the judge on motion or otherwise shall alter the time. Here there were numerous requests from both the plaintiff and the defendants for what were in effect enlargements of time for filing of the master’s report. Each party’s request was assented to by the other party, and each was allowed by a judge of the Superior Court. The report was filed within the time allowed in response to the last of those requests.5
4. The defendant argues that a photostatic copy of a statement signed by Melanson was improperly received in evidence because of the best evidence rule and the fact that the signature was not authenticated. Melanson, however, acknowledged his signature on other exhibits in evidence and the jury were competent to draw their own inferences as to the authenticity of the offered writing by comparing it to the signatures which Melanson had admitted as genuine. Noyes v. Noyes, 224 Mass. 125, 130 (1916).
The “best evidence” rule is preferential, not exclusionary. Fauci v. Mulready, 337 Mass. 532, 540 (1958). Secondary evidence may be introduced if, as here, the judge finds that the original writing is unavailable through no seri*331ous fault of the proponent. Id. at 542. See also Old Colony Trust Co. v. Shaw, 348 Mass. 212, 219 (1964). The decision to admit the letter and let the jury give it the weight they deemed justified was a matter within the judge’s discretion and was fully justified on the evidence before him. Nunes v. Perry, 113 Mass. 274, 276 (1873). Pataskas v. Judeikis, 327 Mass. 258, 259-260 (1951).
Judgment affirmed.
Order denying motion for new trial affirmed.