226 



PHO TO-MICROGRAPHY 



[ CH. VIII 



tion 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 oculars in photography, although excel- 

 lent 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.* 



No. a. 



FIG. 185. Projection Oculars with section 

 removed 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. ) 



$ 366. Difference of Visual and Actinic 

 Foci. Formerly there was much difficulty ex- 

 perienced in photo-micrographing on account 

 of the difference in actinic and visual foci. 

 Modern 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 arisen from differences 

 in the foci. This is especially true when mono-chromatic light and even when 

 petroleum light is used. In case the two foci are so unlike in an objective, it 

 would be better to discard it for photography altogether, for the estimation of the 

 proper position of the sensitive plate after focusing is only guess work and the 

 result is mere chance. If sharp pictures cannot be obtained with an objective 

 when petroleum light and orthochromatic plates are used the fault may not rest 

 with the objective but with the plate holder and focusing screen. They should be 

 very carefully tested to see if there is coincidence in position of the focusing 

 screen and the sensitive film as described in \ 362. 



367. Apparatus for Lighting. For low power work (35 mm. and longer 

 focus) and for large objects, some form of bull's eye condenser is desirable 

 although fairly good work may be done with diffused light or lamp-light reflected 

 by a mirror. If a bull's eye is used it should be as nearly achromatic as possible. 

 The engraving glass shown in Fig. 188 answers well for large objects. For smaller 



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

 was made with the achromatic objective 25 mm. (i inch), 18 mm. (finch), 5 

 mm. (\ to \ inch) and 2 mm. ( T V inch) homogeneous immersion of the Bausch & 

 Lomb Optical Co.; Gundlach Optical Co.; Leitz ; Reichert ; Winkel, Zeiss and the 

 Spencer Lens Co. Good results were obtained with all of these objectives both 

 with and without projection oculars. 



