SPECIAL PHOTOMICROGRAPHY 33 



If preferred, focusing may be done with a lens; 

 but in the case of a specimen of blood, the edges of 

 the red cells afford a good indication of its accuracy, for 

 they seem just to disappear when the accurate focus 

 is obtained. When they are out of focus the edges of 

 the cells stand out in high relief. Having obtained the 

 focus and stress must be laid on this point the cell 

 or other object is deliberately thrown out of focus to the 

 extent of about erirth of a millimetre 1 by screwing down 

 the fine adjustment so as to bring the objective nearer 

 the object. The reason for this is that the cells are 

 resting on a jelly under a cover-glass which is all the 

 time slowly sinking into the jelly, and, of course, 

 carrying the cells with it. The latter, therefore, are 

 sinking out of focus all the time. By deliberately 

 "over-focusing," when the exposure is actually made 

 the focus will become accurate, and the sinking of 

 the cover-glass compensated for. 



The length of the exposure varies with the objective 

 used and the candle-power of the light, which in its 

 turn varies with the voltage. It is best to find the 

 length of the exposure by experiment, but we give 

 about twenty seconds with the apochromatic objective, 

 using "backed" Imperial plates. The water cooling 

 tank cuts out light, but it is very necessary to use it 

 in order to delay death of the cells and the onset of 

 achromasia, both of which are accelerated by heat 

 rays. The tank cuts off some of the heat rays, but 

 allows the passage of the actinic ones. Many specimens 



1 The fine adjustments of most microscopes are graduated to allow of this 

 measurement. 



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