CH. Villi PHOTO-MICROGRAPHY 231 



It will be necessary to focus down considerably to make the 

 image clear. Lengthen or shorten the bellows to make the image 

 of the desired size, then focus with the utmost care. In case the 

 field is too much restricted on account of the tube of the microscope, 

 remove the draw-tube. When all is in readiness it is well to wait 

 for three to five minutes and then to see if the image is still sharply 

 focused. If it has become out of focus simply by standing, a sharp 

 picture could not be obtained. If it does not remain in focus, some- 

 thing is faulty. When the image remains sharp after focusiug make 

 the exposure. From 20 to 60 seconds will usually be sufficient time 

 with medium plates and light as described. If a color screen is used 

 it will require 50-300 seconds, i. e., 2 to 5 times as long, for a 

 proper exposure (§ 294). 



B. Photographing with a Projection Ocular. — If the object is 

 small enough to be included in the field of a projection ocular (Fig. 

 172) use that for making the negative as follows : Swing the camera 

 around so that it will leave the microscope free. Use an ordinary 

 ocular, focus and light the object, then insert a projection ocular in 

 place of the ordinary one, and swing the camera back over the 

 microscope. It is not necessary to use an ordinary ocular for the 

 first focusing, but as its field is larger it is easier 'to find the part to 

 be photographed. The first step is then to focus the diaphragm of 

 the projection ocular sharply on the focusing screen. Bring the 

 camera up close to the microscope and then screw out the eye-lens 

 of the ocular a short distance. Observe the circle of light on the 

 focusing screen to see if its edges are perfectly sharp. If not, con- 

 tinue to screw out the eye lens until it is. If it cannot be made 

 sharp by screwing it out reverse the operation. Unless the edge of 

 the light circle, i. e., the diaphragm of the ocular, is sharp, the re- 

 sulting picture will not be satisfactory. 



It should be stated that for the X 2 projection ocular the bellows 

 of the camera must be extended about 30 or 40 centimeters or the 

 diaphragm cannot be satisfactorily focused on the screen. The X 4 

 projection ocular can be focused with the bellows much shorter. 

 For either projection ocular the screen distance can be extended 

 almost indefinitely. 



When the diaphragm is sharply focused on the screen, the 

 microscope is focused as though no ocular were present, that is, first 

 with the unaided eye then with the focusing glass, The exposure 



