1 44 PHOTOMICROGRAPHY 



best. I A camera extension of 10" will give a photomicro- 

 graph of similar amplification to that observed directly 

 in the microscope. Variations in camera length give 

 proportional variations in magnification. 



4. Examine the effect produced by closing the iris 

 diaphragm, and decide on the best aperture to employ ; 

 it should be kept as large as possible, as diffracion effects 

 which are easily overlooked become distinct in a photo- 

 micrograph. 



5. Place various colour screens in the substage ring or 

 between the lamp and mirror, and select the most suit- 

 able, as explained in Chapter IX. 



6. "Try the effect of dark ground and oblique illumina- 

 tion (Chapter XI), but use the latter with great caution 

 as it may produce false images. 



Besides this careful examination of the specimen there 

 are various optical details that are best attended to before 

 the microscope is transferred to the photomicrographic 

 stand. Of these a correct centration of condenser and 

 objective has already been mentioned. For high-powers 

 it is necessary to consider the thickness of the cover-glass, 

 and make any adjustment of the correction collar or tube 

 length that may be found desirable, as explained on 

 page 69. And above all, the lenses and slide must be 

 examined for any specks of dirt, and, if found, their loca- 

 tion on objective, eyepiece, or condenser may be decided 

 by turning each of these round in turn and noticing 

 whether the specks move. The dirt is then removed by a 

 silk handkerchief or soft tissue-paper. 



Having decided whether the object is to be photo- 

 graphed as a whole or in part, and what magnification 

 and method of illumination best display the necessary 

 detail, attention must be directed to the camera and 

 optical bench, and such dispositions made as will best 

 attain the desired results. Systematic instructions are 

 given for obtaining critical illumination and an even 



