252 PEACTICAL PHOTO-MICEOGKAPHY 



may therefore be used alternatively. In the searcher eye-piece 

 is a small fluorescent screen, and a high-power magnifier is 

 arranged so that the image formed on this screen may be 

 directly observed. After the image has been adjusted exactly 

 on the fluorescent screen, it will then be in focus when the 

 camera is placed in position and the photographic plate 

 substituted. Variations in the length of the camera, not 

 exceeding a few centimetres, will not have any considerable 

 effect upon the sharpness of the image. 



The source of light is supplied by passing a high-tension dis- 

 charge from a coil and a Ley den jar between cadmium elec- 

 trodes. The actual production of the necessary high-tension 

 current may be by means of an induction coil giving a spark, 

 say, about three inches in length. The light emitted by the 

 electrodes is split up, by special illuminating apparatus consist- 

 ing of lenses and prisms of quartz ; the light of 275 /*//. which 

 is emitted from the cadmium electrodes, or of 280 fifi which 

 may be used alternatively from magnesium electrodes, is 

 spread out into a spectrum, and the portion not required is cut 

 off by means of an iris-diaphragm. This diaphragm allows 

 a beam to pass which is collected by a condenser of quartz 

 taking the place of the ordinary sub-stage condenser of the 

 microscope ; the rays are thus focussed with a narrow or wide 

 cone, as required, on to the object and pass into the objective. 



Table V gives the range of magnification that may be 

 obtained, and it will be seen that there is almost as much 

 range as would be obtained with a battery of ordinary objectives. 

 The optical camera-lengths are given in centimetres, as with 

 ordinary objectives. To obtain exactly a given magnification, 

 it is advisable to use a stage-micrometer, altering the camera- 

 length until the exact magnification required is obtained. 



It will be appreciated that in addition to the increased 

 resolving-power, this method is in effect another means of 

 differential staining. All bodies to be photographed by this 

 means must be unstained, and any detail of structure will be 

 brought out as the result of differences in the absorptive-power 

 of the tissue to ultra-violet light. The preparations must 

 not be fixed, dried, or otherwise treated in the way that 

 microscopical preparations are usually dealt with. It is also im- 

 portant to remember that the medium in which the preparation 



