72 PRACTICAL PHOTO-MICROGRAPHY. 



for medium and high power work ; but it is not so, for the diffi- 

 culty of focusing the image of the ground glass again comes in. 

 It may be worth while to try, in place of ground glass at G, a cell 

 full of milk and water, as suggested by a friend of the writer. 

 There is still another method of obtaining, with a condenser 

 alone, an evenly lighted field. This method consists in mis- 

 focusing the condenser, but we need hardly point out at any 

 great length the danger of this system. If, in Fig. 27, A be 

 the object and C the condenser focused on A, as shown by 



A ^! 



FIG. 27. 



continuous lines, the image of the light seen with a low-power 

 objective may be too small to cover the field ; if we focus the 

 condenser down (dotted lines), or up (interrupted lines), we 

 shall get an even field of light, but our object will no longer 

 fulfill our condition of being in the foci of condenser and 

 objective at once, and so our image will be inferior. If we 

 focus our condenser and o. g. both on our object, look down 

 the tube without ocular, arrange our condenser aperture so as 

 to file our o. g. with light, and then rack down our condenser, 

 we shall see that our o. g. is no longer fully utilized ; but on 

 replacing the ocular we may find our field now evenly lighted. 

 So that focusing down our condenser has entailed loss of 

 aperture. If, by opening our condenser aperture by an iris 

 or by a larger stop we can once more fill our objective with 

 light, probably not much harm will be done to the quality of 

 image by our racking down of the condenser ; still the writer 

 decidedly objects in practice to this system of mis-focusing, 

 and recommends any other system in preference to this one. 



To recapitulate : With low powers, where the image of the 

 radient focused on the object does not sufficiently fill the field, 



