('//. 



223 



amplifier. The introduction of oculars especially designed for projection, has 

 led to the discarding of ordinary oculars and of amplifiers. Oculars restrict 

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

 large specimens.* 



No. 2. 



No. 4. 



FIG. 172. Projection Oculars with section 

 removed to shozv (lie construction. Below arc 

 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- 

 ject ire as with the compensation oculars, (Zeiss' 

 Catalog. ) 



301. Difference of Visual and Actinic Foci. Formerly there was much 

 difficulty experienced 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 an objective has its visual and actinic foci at markedly different 

 levels it would be better to discard it for photography altogether, for the esti- 

 mation 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 \ 296. 



ic 302. 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. 175 answers well for large 



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

 was made with the achromatic objective 25 mm. (I inch), 18 mm. ( ^ inch), 5 

 mm. (i to |g inch) and 2 mm. ( T 'j 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. 



