52 The Microscope 



with a groove into which may be fitted either a piece of ground glass or a 

 film holder. 



The advantages of this arrangement are in favor of quality rather than 

 convenience. Particularly, it is possible to use special photographic oculars 

 and, in the equipment shown in Fig. 64, to vary the distance of the film 

 from the ocular. This permits the selection of specific areas of the field 

 and also gives a flatter field than other methods. If the reader will re- 

 examine Fig. 1, he will observe that the visual field is curved and that 

 the rays leave the top lens of the ocular at a very wide angle. It follows 

 that a flat film placed above a visual ocular will be out of focus at the 

 edges and, unless it is placed very close, will show an excessively magni- 

 fied image. 



Every manufacturer of microscopic equipment offers a special photo- 

 graphic ocular in which Ramsden's disc is formed several inches above 

 the top lens so that the image projected onto the film will create the im- 

 pression of being the same size as that viewed in the eye. These oculars, 

 moreover, are designed to project a flat, rather than a curved image. The 

 author prefers, of all the makes at present available, the series offered by 

 Reichert. The photographic oculars of this company have the top lens 

 fitted in a spiral focusing mount and may therefore be individually ad- 

 justed for the exact projection distance required. 



The disadvantages of this type of equipment are very great. It is 

 necessary to use a vertical monocular microscope. Then the camera must 

 be swung into position and lowered into place. Next, final focusing must 

 be carried out by observation of the image on a ground-glass plate; this 

 is both difficult and inconvenient. Finally the ground glass must be re- 

 moved and a film substituted. All this is so laborious and so time-consum- 

 ing that, save for the most critical work, the disadvantages outweigh the 

 advantages. 



Reflex-image Projection ( Figs. 62 and 65 ) . A vertical monocular micro- 

 scope is set up and a roll-film camera with a built-in shutter is substi- 

 tuted for the eyepiece. The camera has a short horizontal tube into which 

 the image can be deflected for focusing. 



The advantages of this type of camera are that it is less trouble to set 

 up than the type just discussed. The deflected image is arranged to come 

 to a focus on a disc of glass bearing cross hairs. The focusing eyepiece is 

 adjusted to give a sharp image of these hairs, and the fine adjustment of 

 the microscope is then used to bring the image into focus; swinging the 

 mirror aside will then cause a sharp image to be projected onto the film. 



The disadvantages of this type of camera are in its lack of flexibility. 

 The ocular is built into the tube of the camera and is so designed that 

 the visual field of view will fill the whole area of the film, no matter what 



