Ch. VII] PHOTOGRAPHING WITH THE MICROSCOPE 225 



board screen, as described above (§ 334). The latter method is like 

 focusing with the compound microscope and a positive ocular. If 

 the focusing glass is set properly the focus should be easily and accu- 

 rately determined. 



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 one 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. (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 cannot get a good picture. In that case 

 it is necessary to study the apparatus and see which part of the 

 mechanism is at fault. 



§ 349. Photo-micrographs of 20 to 50 diameters. — For pictures 

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

 with the objective in the end of the camera, and the special micro- 

 scope stand for focusing (fig. 126). 



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

 a low objective, 20 to 35 mm. equivalent focus, and no ocular (fig. in). 

 The object is placed on the stage of the microscope and focused as 

 in ordinary 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. 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 placing 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 (§ 366). 



For a horizontal camera it is frequently better to swing the mirror 

 entirely out of the way and allow the light to enter the condenser 



