CM. VIII. ] 



PHO T0-3IICR0GRAPH Y. 



189 



§ 328. Objectives and Oculars for Photo-Micrography. — The belief is almost 

 universal that the apochromatic objectives are most satisfactory for photography. 

 They are employed for this purpose with a special projection ocular. Two very 

 low powers, one of 35 and one of 70 millimeters equivalent focus are used without 

 any ocular (Fig. 149). Some of the best work that has ever been done, however, 

 was done with achromatic objectives (work of Woodward and others). One need 

 not desist from undertaking photo micrography if be has good achromatic ob- 

 jectives. From a somewhat extended series of experiments with the objectives of 

 many makers the good modern achromatic objectives were found to give excellent 

 results when used without an ocular. Most of them also gave good results with 

 projection oculars, although it must be said that the best results were obtained with 

 the apochromatic objectives and projection oculars. It does not seem to require 

 so much skill to get good results with the apochromatics as with the achromatic 

 objectives. The majority of photo-micrographers do not use the Huygenian ocu- 

 lars in photography, although excellent results have been obtained with them. An 

 amplifier is sometimes used in place of an ocular. Considerable experience is 

 necessary in getting the proper mutual position of objective and amplifier. The 

 introduction of oculars especially designed for projection, has led to the discarding 

 of ordinary oculars and of amplifiers. However the projection oculars of Zeiss 

 restrict the field very greatly, hence the necessity of using the objective alone for 

 large specimens.* 



Fig. 145. Projection Oculars with section re- 

 moved to show the construction. Below are 

 shown the upper end with graduated circle to 

 indicate the amount of rotation found necessary 

 to focus the diaphragm on the screen. No. 2, 

 No. 4. The numbers indicate the amount the 

 ocular magnifies the image formed by the ob- 

 jective as with the compensation oculars. 

 {Zeiss' Catalog, No. 30). 



\ 329. Difference of Visual and Actinic Foci. 

 Formerly there was much difficulty exper- 

 ienced in photo-micrographing on account of 

 the difference in actinic and visual foci. Mod- 

 ern objectives are less faulty in this respect 

 and the apochromatics are practically free 

 from it. Since the introduction of orthochromatic or isochromatic plates 

 and, in many cases the use of colored screens, but little trouble has 



*The comparative study both with projection oculars, and without an ocular were 

 made with the achromatic objectives 25 mm. (1 inch), 18 mm. (X inch), 5 mm. (| 

 to J inch) and 2 mm. jV (inch) homogeneous immersion of the Bausch & Lomb 

 Optical Co. ; Gundlach Optical Co.; Leitz ; Reichert ; Winkel and Zeiss. Good 

 results were obtained with all of these objectives both with and without projection 

 oculars. The objectives of Spencer and Smith, especially constructed for photo- 

 micrography, are highly spoken of by Piffard (Jour. Roy. Micr. Soc, 1892-1893). 

 The writer has not had the opportunity of comparing these with those mentioned. 

 Piffard spoke highly of the work of Wales also. From the known excellence of 

 the work of these opticians one would expect good results. 



