1893.] *J^^ [Oliver. 



with their convex sides placed outwardly. He made the outer meniscus 

 one-half larger than the back lens, thus allowing six different combina- 

 tions with seven single lenses. Such a lens having an angle of nearly 

 ninety degrees and great depth of focus, and giving extreme sharpness 

 over the whole field, and being free from all distortion, necessarily became 

 a most excellent instrument for architectural work and copying. 



The story of the invention is this: One year before the patent of the 

 lens was obtained, Mr. Coleman Sellers, who was at that time greatly 

 interested in photography, requested Mr. Zentmayer to explain the theory 

 of the then favorably known " Globe lens." Whilst examining a sample 

 of the lens, Mr. Zentmayer remarked, "Why did the inventor adopt an 

 acliromatic combination, when the same or even better results could have 

 been obtained by the combination of two simple meniscus lenses? Recog- 

 nizing the force of the query, Mr. Sellers requested Mr. Zentmayer to ex- 

 periment with a double lens. This liaving been successfully accom- 

 plished, he urged him to apply for a patent, which was reluctantly agreed 

 to and done after the most earnest solicitation. 



The introduction of this lens engendered a most spirited controversy as 

 to question of the theory in optics involved in its construction. Prof. 

 Henry Morton, Dallmeyer and other well-known authoritative scientists 

 and experts, both here and abroad, took part in this discussion. Mr. 

 Zentmayer's personal appearance in the matter, which showed itself in a 

 short article entitled "Refraction without Dispersion and some Reflec- 

 tion," in the August, 1867, number of the Philadelphia Photographer, 

 proved at once in a most forcible and logical manner that the writer was a 

 consummate master in the field of theoretical and applied optics ; a paper 

 that immediately established him as America's foremost optician. 



One monograph, as further illustrating the I'cmarkable clearness, ease of 

 expression and fullness of comprehension with which he surrounded one 

 of the most abstruse and most readily misunderstood of the theoretical and 

 applied sciences — optics — is his illustrated brochure of twenty-three pages 

 entitled " A Lecture on Lenses." This, which appeared in the May and 

 June, 1876, numbers of the Journal of the Franklin Institute, is even now 

 authoritatively recognized as one of the best, the most concise, and the 

 clearest expositions of tlie subject that has ever been presented to the 

 public. 



Engaged as the official maker of the microscopes used in the hospitals of 

 the United States Army ; appointed a member of tlie Iowa Total Eclipse 

 Exhibition in 1869, to the success of which he contributed largely by the 

 device of some of the most delicate of the photographic machinery ; a 

 member of the Judge of Awards in the 1874 Fair of the Franklin Insti- 

 tute, the International Centennial Exhibition in 1876, and the Electrical 

 Exhibition in 1885 ; a life-member of the German Hospital, and a member 

 of the American Philosophical Society, the Academy of the Natural 

 Sciences of Philadelphia, both the American and British Associations for 

 tlie Advancement of Science, the Franklin Institute of Philadelpliia, both 



