228 PHOTO-MICROGRAPHY [CH. VIII 



(B) . To enable the operator by looking directly into the micro- 

 scope to focus correctly for any distance of the photographic plate 

 (length of bellows), Foot and Strobell introduced the use of concave 

 spectacle lenses ranging from — i D to — 10 D. ( — i to — 10 diopters). 



They have produced some of the best photo- micrographs of 

 recent years by their method. (See for the full account, Zeit. wiss. 

 Mikroskopie, Bd. 18, pp. 421-426; Jour. Ap. Microscopy, Vol. V. 

 1902, p. 2082). 



In whatever way one focuses for photo-micrography a difficulty 

 often appears. No matter how perfect the focus of the microscope 

 the picture may be out of focus. This may be due to either of two 

 things : (1) the focusing screen or focusing glass may not be in the 

 right position to make the image sharp on the sensitive plate 

 (§ 282, 296). (2) The microscope may get out of focus while the 

 picture is being made. The reason for this change may be the 

 gradual settling down of the tube of the microscope. This may be 

 a fault of the fine or of the coarse adjustment. It is a good plan to 

 focus the object carefully and after 10 or 15 minutes to see if the 

 focus is still good. If the microscope will not stay in focus one can- 

 not get a good picture. In that case it is necessary to study the 

 apparatus and see which part of the mechanism is at fault. 



§ 307. Photo-micrographs of 20 to 50 Diameters. — For 

 pictures under 15 or 20 diameters it is better to use the camera for 

 embryos with the objective in the end of the camera, and the special 

 microscope stand for focusing (Fig. 165). 



For pictures at 25 to 50 diameters one may use the microscope 

 with a low objective, 25 to 35 mm. equivalent focus, and no ocular 

 (Fig. 180). The object is placed on the stage of the microscope, 

 and focused as inordinary observation. If a vertical microscope is 

 used the light from the petroleum lamp or other artificial light, is 

 reflected upward by the mirror. It may take some time to get the 

 whole field lighted evenly. Refer back to § 106 for directions. In 

 some cases it may be advisable to discard the condenser and use the 

 mirror only. For some purposes one will get a better light by plac- 

 ing the bull's eye or other condenser between the lamp and the 

 mirror to make the rays parallel or even to make a sharp image of 

 the lamp flame on the mirror. Remember also that in many cases 

 it is necessary to have a color screen between the source of light and 

 the object (§ 291)'. 



