190 PEACTICAL PHOTO-MICBOGKAPHY 



construction is fitted and this is really an unnecessary and 

 expensive luxury it will be found that some vibration is 

 bound to take place. 



It should be mentioned that when using very low powers 

 the blackened card or its substitute may be interposed between 

 the microscope objective and the object, thereby minimising 

 to a considerable extent the chance of reflected light entering 

 the apparatus. It is obvious that such reflections are more 

 likely to occur with low-power lenses than with high-power 

 ones, since the former transmit so much more light. 



Various methods for effecting the exposure will suggest 

 themselves to the worker, and any one may be adopted that 

 does not result in disturbance of any part of the apparatus, 

 and yet ensures that the sensitive plate is protected against 

 stray light. 



Focussing. The various appliances for properly focussing 

 the image have been already described, but there are one or 

 two further points that may arise in the course of their use. 

 It is of course necessary that any worker should first become 

 conversant with the use of the fine-adjustment on the micro- 

 scope under the ordinary conditions of visual work, and it is 

 assumed that this has been already mastered. 



In photo-micrographic work the fine-adjustment is usually 

 actuated by means of some long focussing-rod, which the 

 observer can rotate at one end, and which is connected at the 

 other end to the fine-adjustment milled head on the microscope 

 itself. The various types of these appliances have already 

 been fully detailed. In actual practice, whichever focussing 

 arrangement is adopted, it should act on the fine-adjustment 

 with precision. It will be seen in general that one of the 

 arrangements, in which thin cords are taken over the fine- 

 adjusting head very lightly, are in practice preferable. The 

 arrangement should be such that the image only moves in and 

 out of focus when the focussing-rod is rotated, and so that no 

 lateral displacement of the image occurs. The rigid method 

 of focussing adopted in the Zeiss model, in which the motion is 

 imparted to the fine-adjustment by means of a Hooke's joint, 

 usually causes some slight lateral movement of the image 

 when used with high powers, although it must be admitted 

 that the actual focussing can be done with great exactitude. 



