ing this accused of his constitutional right to cross-examine the prosecution’s witnesses on so important a link in the case against him.
The right to cross-examine is basic in our judicial system (Alford v. United States, 282 U. S. 687; People v. Becker, 210 N. Y. 274, 304-305; People v. Cole, 43 N. Y. 508, 512-513), and has been from earliest times (Matter of Friedel v. Board of Regents, 296 N. Y. 347, 352). Although in the last-cited case we held that “ once the right has been accorded, the extent of cross-examination rests largely in the discretion of the tribunal, whose exercise thereof is not reviewable unless abused ”, such a denial or limitation of cross-examination has been upheld only where it has related to collateral matters, or where the right itself has been abused (Matter of Friedel v. Board of Regents, supra, p. 352; People v. Braun, 158 N. Y. 558, 567-569; McGuire v. People, 48 How. Prac. 517; White v. McLean, 57 N. Y. 670, 671, opinion in 47 How. Prac. 193; Langley v. Wadsworth, 99 N. Y. 61, 63). Surely a Trial Judge cannot deprive a defendant of his constitutional right to inquire where, as here, the matter is directly relevant to the principal issues of the case against him (People v. Becker, supra; Alford v. United States, supra; People v. Cole, supra).
Even if the information in question were in any sense privileged, the People abandoned any such privilege by adducing on direct examination the discrepancy between the two motor numbers (United States v. Andolschek, 142 F. 2d 503, 506; United States v. Beekman, 155 F. 2d 580, 584; United States v. Keown, 19 F. Supp. 639; United States v. Coplon, 185 F. 2d 629, 638, certiorari denied 342 U. S. 920; see People v. Schainuck, 286 N. Y. 161; Gordon v. United States, 344 U. S. 414). One may not “ fill a gap in his own evidence by recourse to what he suppresses ” (United States v. Coplon, supra, p. 638; see, also, United States v. Reynolds, 345 U. S. 1, 12).
Of course the insurance companies and the police are free to use such confidential motor numbers in conducting their auto theft investigations. But in a court of justice they either must employ some other method of identifying the stolen ear — as indeed they could have confined themselves to in this case — or, if the confidential number is used as evidence against the defendant, they must permit their witnesses to be cross-examined with