PRACTICAL PHOTO-MICROGRAPHY. 5 



up " carried on, modified, and in many cases and ways cor- 

 rected by an ocular. Still the author has seen and produced 

 micrographs without an ocular not easily to be surpassed. 



One of the latest outcomes of optical science has been the 

 construction by Herr Zeiss, of Jena, on formulae by Dr. Abbe, 

 of a series of oculars arranged for the purpose of projecting 

 the image formed by the objective. The writer may as well 

 state at once that he believes, and has good reason to believe, 

 that in the use of the projection oculars, with other matters of 

 different nature, will lie the future of scientific photo-micro- 

 graphy. In the author's experience these oculars have acted 

 well with objectives made by makers other than Zeiss ; for in- 

 stance, they have been found to work satisfactorily with a 

 1-inch, a -J-, and a T ^- immersion, all by Swift of London ; also 

 fairly well with a -^ by Reichert. It is matter for congratu- 

 lation that the Zeiss (or Abbe) projection oculars are cheap, 

 costing only about 2, or say $10 each. A chapter will 

 be devoted to the apochromatic lenses and " compensating " 

 and " projection " oculars made by Zeiss. 



Undoubtedly an eye-piece of any kind, when it can be used 

 without detriment to result, is a great convenience. The cam- 

 era does not require to be so long ; there is less danger of 

 internal reflections, which must be sedulously avoided, as will 

 presently be seen. But failing a projection ocular, or failing 

 the necessary coincidence between projection ocular and ob- 

 jective, the balance of opinion and the balance of high-class 

 results are probably in favor of the image projected directly 

 upon the screen by the objective. The writer at all events is 

 willing to commit himself to this opinion, and to recommend 

 either a projection ocular or none. 



Amount of Magnification. To discuss this question we may 

 adopt an arbitrary term : " Initial power." We propose to 

 call the initial power of an objective the amount of magnifica- 

 tion (in terms of diameters) given by the objective at 10 inches 

 behind its posterior conjugate focus. (The distance, 10 inches, 

 is about the distance for distinct normal vision, and is chosen 

 for that reason.) A good objective will, as a rule, when properly 

 used, stand a strain of magnification to ten times its initial power. 



