The defendant was convicted of so driving an automobile that the lives and safety of the public might be endangered. The testimony offered by the Commonwealth tended to prove that an automobile, while being driven by the defendant in Adams Square, Boston, at the rate of thirty miles an hour, hit a woman who was crossing the street. Subject to an exception by the defendant, the daughter-in-law of this woman was permitted to testify that the condition of her mother-in-law, as to her ability to walk, her hearing and eyesight, was good. The questions called for no expert opinion but for such information as is obtained by ordinary observation. Commonwealth v. Sturtivant, 117 Mass. 122. It was competent for the Commonwealth to prove all the circumstances, including the condition, movements and conduct of the person injured. Commonwealth v. Vartanian, 251 Mass. 355. The testimony also was *74proper for the consideration of the jury in connection with that of the defendant offered later, that the woman walked into the left mudguard of the automobile. The other exception to the admission of evidence not having been argued is treated as waived.
Counsel for the defendant, in cross-examining a witness called by the Commonwealth, asked: “Did you discuss this case with any one since the accident?” Upon objection by the district attorney, counsel stated, in reply to an inquiry by the trial judge, that his purpose in asking the question was to show that the witness had assisted the government in looking for witnesses and in prosecuting the case, and to disclose that the witness was prejudiced. The trial judge excluded the question. The defendant then asked the witness, “Did you spe'ak to a newsboy in Adams Square with reference to this case?” Upon the statement of counsel for the defendant that that question was asked for the same purpose, the judge excluded it. The credit to be given a witness to material facts is not merely collateral, and reasonable cross-examination for the purpose of proving the falsity of his testimony or bias or prejudice on his part has been held to be a matter of right. Day v. Stickney, 14 Allen, 255. Stevens v. Stewart-Warner Speedometer Corp. 223 Mass. 44. Commonwealth v. Russ, 232 Mass. 58, 79. The answers to the two questions asked would not in themselves have a tendency to show either of these things, and upon the offer of proof it is not possible to determine what facts counsel expected to bring out and whether they, if established, would have a tendency to prove interest or prejudice. The offer did not suggest that the witness was employed or asked by the Commonwealth to assist in the case, that he had any pecuniary interest in it, nor that he had expressed any ill will toward the defendant. It did not appear from the offer that the witness said anything to anyone who did or could assist the Commonwealth, and the witness was not asked if he had assisted the Commonwealth in any way. The offer was broader than the questions and in large part not responsive to them. Hallwood Cash Register Co. v. Prouty, 196 Mass. 313, 315. The only witnesses called were *75the witness being cross-examined, the daughter-in-law who accompanied the woman who was injured, the defendant, and a witness called by him. No newsboy or other witness was called either to testify or to prove that he was asked to testify. The burden is on the party excepting to show that he has been harmed by a ruling. McGonigle v. Belleisle Co. 186 Mass. 310. Commonwealth v. Phelps, 210 Mass. 109, 114. It is not enough to state the purpose of an offer of this kind in such general terms that it does not disclose to the court the facts which, if established, would tend to show interest or prejudice. Ford v. Ford, 104 Mass. 198, 206. Prejudices are usually indicated by some form of expression. Day v. Stickney, supra. The witness being cross-examined was one on whom the Commonwealth relied in part to prove the commission of the crime. Such a witness has the right, if not the duty, to assist at least to the extent of disclosing to the prosecuting attorney all facts within his knowledge material to the case, including the names of witnesses known to him. If he does no more than this he cannot fairly be charged with partisanship. The defendant has failed to show that the discretion of the trial judge was not properly exercised in excluding the questions asked.
The exception of the defendant to a part of the charge in which the judge quoted from the opinion in a decided case must be overruled. Commonwealth v. Dow, 217 Mass. 473, 483. This exception was taken to a portion of the charge which covered nearly a page of the printed record, and which in its main features stated the principles of law applicable to the facts of the case. The attention of the judge was not called to any particular sentences or phrases in the paragraphs excepted to, and which the defendant now contends are objectionable. Lockwood v. Boston Elevated Railway, 200 Mass. 537, 544. Draper v. Cotting, 231 Mass. 51, 63. It is not error for the trial court in the course of a charge to state the legislative purpose in enacting the statute upon which the indictment is based.
Exceptions overruled.