Ifi-JO Journal of Applied Microscopy 



For this class of work it is therefore necessary to make a special arrangement 

 and record it. 



The field to be taken must of course be evenly illuminated ; this will be so 

 nearly secured by the arrangements mentioned above that it can be completed 

 by looking the field over on the ground glass and slightly moving the light. 



The best work requires the illumination of the object by light of nearly the 

 same wave length. This is easily accomplished by the use of Zettnow's filter. 

 Nearly all photographic houses sell screens of some sort ; a lantern slide fixed 

 without exposure and stained with tropajolin is recommended ; we have tried 

 several of these substitutes, but find nothing better than Zettnow. 



The solution is prepared as follows: 1(5U grams — pure dry copper nitrate; 

 14 grams — pure chromic acid ; water to make up '250 c. c. The color screen 

 should be one cm. thick. 



In the illumination of the object it is essential that the cone of light bear the 

 proper relationship to the numerical aperture of the objective if refraction and 

 not color absorption is to produce the image. A cone too wide will dim the 

 object (fog the negative) or in some cases entirely drown it. A cone too narrow 

 will in all cases utterly ruin it with diffraction halos. This fault is the common- 

 est in photomicrography for the reason that a small cone of light minimizes 

 focus difference and gives a sharp appearance to all parts of the object on the 

 ground glass. This appearance is, however, not a faithful picture of the object. 



The cone of illumination should be equal to the cone entering the objective ; 

 the wider therefore its angular aperture the wider the diaphragms should be 

 opened. The best practical ways I know to ascertain the proper diaphragm in 

 a given case, is to take out the eyepiece and, looking into the microscope tube, 

 reduce the diaphragm until a bright circle of light fills about one-third of the 

 objective. Another way is to look in without removing the eyepiece through a 

 hand lens such as one uses in focusing on the ground glass. The light must of 

 course be reduced with ground glasses or color screens before one ventures to 

 look at it in either of these ways. 



In the photography of colored objects, as bacteria (Fig. 3), in which absorp- 

 tion in different degrees produces the image, the diaphragm may without detri- 

 ment be opened wide. D. W. Denni.s. 

 Earlham College. 



The Palasade Mfg. Co., of Yonkers, New York, have issued two small pam- 

 phlets for free distribution among physicians, which may be found useful. One 

 is entitled " The Essentials of Hematology " and contains careful directions for 

 the study of blood in the detection of disease. The second is " A Syllabus of 

 Bacteriology." This is a simple account of the facts of bacteriology which bear 

 on practical medicine, together with descriptions of methods of bacterial study. 

 The work is clear, is illustrated by rather gorgeous colored plates, and will be 

 found useful to physicians who are interested in developing the practical side of 

 bacteriological technique. Either of these pamphlets may be obtained from the 

 publishers. ii. w . c. 



