1034 Transactions of the AuMEeican Institute. 



meniscus lens costino; one dollar. In makiuo; the ncjrative he had 

 used a diaphragm with one-fourth inch opening in front of the lens, 

 and giving about one minute exposure. The print exhibited was 

 very good, and occasioned considerable discussion upon the pro- 

 duction of good results through what was commonly believed to 

 be inferior means. 



The Secretary gave a description of the focimeter which he used 

 for testing the variation between the visual and chemical foci of 

 lenses used for photographic purposes. The device consisted of 

 a block of wood presenting the surface of an acute angle towards 

 the lens under examination; upon the face of this angle was placed 

 finely printed matter so arranged in combination with a scale read- 

 ing each way from zero, as to admit of a test to the tenths or 

 hundredths of an inch, and thus fixing the exact plus or minus 

 length of the chemical focus, when compared with the visual, at 

 any and all reductions or enlargements of the image. 



Then followed a general discussion upon the relative merits of 

 ammonia sulphurate of iron and plain iron developers. Most of 

 those who had used the two, thought that the ammonia sulphurate 

 of iron gave the finest and most uniform results. 



The Section then adjourned to the first Tuesday in April. 



April 7, 1868. 

 Mr. Charles W. Hull in the chair; 0. G. Mason, Secretary. 



Minutes of the last meeting were read and approved. 



Pi'of. Vanderweyde exhibited a great variety of lenses used for 

 photographic purposes, and explained their peculiarities. Among 

 the number was one, both sides of which had been ground upon 

 the same tool, which had a focus of twelve feet. He also exhibited 

 a combination of prisms, illustrative of the necessity of flint and 

 crown glass for the formation of a perfect acromatised image. 



Mr. Boyle called attention to the recent lens controversy, pub- 

 lished in the PJdladelpJda Photogrcqiher, and asked that a commit- 

 tee be appointed, and instructed to report to a subsequent meeting 

 of the Section, upon the fidelity of his drawings as figured in the 

 Photographer for January last, when compared with those attached 

 to his own and Mr. Zentmayer's patent papers, and his o\\ti illus- 



