54 THE AMEKICAN MONTHLY [March, 



ness, and very courteously showed me their fine work. I had some 

 test objects with me, some balsam-mounted Amphipleuras from Lake 

 Nippersink and from Lake Pistakee, objects which my American homo- 

 geneous immersion objectives, a T 1 „-inch Spencer, N. A. i .38, and a T 1 ? - 

 inch (in reality a ^) Tolles, N. A. 1.30, will resolve with and without 

 sub-stage accessories with comparative ease by lamp-light. These test 

 objects I showed to Mr. Ernst Leitz, to Mr. Seibert, and to Dr. Rod- 

 erick Zeiss. I also showed bromide prints and lantern slides (trans- 

 parencies) of my photographs of Amphipleura pellucida (magnified 

 1 1 20 and 692 diameters) and of Bqcillus anthracis, Bacillus tuber- 

 culosis, Dr. Koch's Comma bacillus, my Bacillus stiis, and several 

 others (taken from the same negatives as those now in your hands for 

 inspection), and all three opticians admitted that they were good, and 

 that my photographs of Amphipleura were the best they had ever seen. 

 Dr. Roderick Zeiss was kind enough to exchange some of his photo- 

 graphs of Amphipleura for mine, and as I have them here with me, I 

 lay them before you that you may judge for yourselves. For further 

 comparison I will also show you some photographs of Amphipleura 

 made in Dr. Koch's laboratory, and one made by Dr. Neuhaus, an ex- 

 pert in photo-micrography. I only have to remark that mine have been 

 made by lamp-light, and the others by sunlight, and with the aid of .a 

 heliostat. Dr. Zeiss's photographs have been made with his apochro- 

 matic -jig-inch homogeneous immersion objectives, N. A. 1.30, his No. 2 

 projection eye-piece, a very ingenious, complicated, and costly camera, 

 and an illuminating apparatus composed of an Abbe homogeneous im- 

 mersion condenser, N. A. 1 .40, and several condensing lenses. A light- 

 filter was also used. In Dr. Koch's laboratory the same appliances are 

 in use. My photographs, on the other hand, have been made in a very 

 simple way. The appliances used by me consist of a Spencer .-j^-inch 

 homogeneous immersion objective, N. A. 1.3S, a common Huyghenian 

 eye-piece (a No. 2 for the higher, and a No. 1 for the lower amplifica- 

 tion), a Bulloch "Professional" microscope stand, a common Blair 

 camera, a coal-oil lamp worth fifty cents, and a condensing apparatus 

 composed of a medium-sized bull's-eye condenser, made by Bausch & 

 Lomb, and an Abbe condenser, made by Bulloch. The frustules pho- 

 tographed by Dr. Zeiss, and in Dr. Koch's laboratory, were mounted in 

 a medium of a refractive index of 2.40 (Stannic chloride), and those 

 photographed by me in a medium of which I do not know the compo- 

 sition and the refractive index, but which probably is not higher than 2 

 or thereabout. 



In at least one respect our first-class American homogeneous immer- 

 sion objectives are preferable. They have collar correction, which is 

 not found in any of the apochromatic homogeneous immersion objectives 

 of German opticians. Our American objectives, therefore, are adapted 

 to a larger range of work, and can be used with any tube-length, while 

 the German apochromatics can not. Still, the latter, it seems to me, 

 are not quite so sensitive to tube-length as is claimed. Further, the 

 German apochromatic homogeneous immersion objectives are more ex- 

 pensive than our American objectives of corresponding quality. So, 

 for instance, apochromatic homogeneous immersion objectives of Zeiss 

 are offered in Jena — a ^-inch, N. A. 1 .30, for 450 marks, or about $1 10 ; 

 a -|--inch, N. A. 1.40, for 550 marks, or about $135 ; a ii-inch, N. A. 



