40 



on tlie optical bench; it would be safer to say that it can be done; when 

 once a combination has been effected that produces this result the exact 

 position of every optical part should be noted carefully. To facilitate 

 this all makers of photomicrographic apparatus would do well to mark 

 a scale on the tables or on the caiTying rods so that all parts can be 

 quickly brought into exactly the same relation to each other and to the 

 object; after many failures and much loss of time in attempting to bring 

 the same state of things to pass that had been previously successful, 

 we had such scales put on our apparatus. Any arrangement of the 

 optical parts will produce an image somewhere; this can be found by 

 carrying a piece of white paper back and forth in the path of the light 

 until the image of the light is found; light and condensers can then be 

 removed until the image rests in the plane of the object to be photo- 

 graphed. In order to have an equally illuminated field it is a good thing 

 to have the size of the equally bright part of the image somewhat larger 

 than the field to be taken; different combinations of the condensers and 

 different positions of these and the light with reference to the object will 

 regulate the size. In work with low powers, 16mm and upwards, this 

 image should fall on the objective instead of the object. If the beginner 

 in his hurry to spoil some plates is satisfied with an approximation to this 

 state of things, or if he lights up and proceeds by the try rule, his time 

 will be lost along with his material. 



(c) The Size of the Entering Cone of Light. 



Three diaphragms should accompany every complete apparatus: One 

 of these, the field diaphragm, should be placed near the double convex 

 lens, and if possible on its microscope side. This must always be used in 

 every exposure; a second is at the focus of the Abbe nearest the source 

 of light, and need not be used when it is swung out; a third is brought 

 on immediately under the object and is consequently open and not in use 

 when the Abbe is; two of the three are accordingly required in every ex- 

 posure, namely, the field diaphragm and the one before or the one behind 

 the Abbe. 



Only a careful study of the effect on the ground glass will avail in all 

 cases for the regulation of these diaphragms. However, two valuable 

 rules can be given: If the Abbe is not in use the diaphragm immediately 

 under the object must be so closed as to cut off all but the field to 

 be photographed; if the Abbe is being used its diaphragm must in general 



