and Laboratory Methods. 



1161 



end of the camera carriage. Here these may be manipulated while the image is 

 being examined and focused on the screen. 



At this stage of the proceedings, there are visible upon the ground glass 

 indistinct images of the incandescent carbon and of the object. By means of 

 the fine adjustments, these are brought into a sharp focus upon the glass. 

 Owing to the nature of the crater, the illumination is not uniform over the whole 

 field and it is necessary to place a piece of ground glass between the source of 

 illumination and the object. This should not be more than dense enough to 

 properly diffuse the light, otherwise it unduly lengthens the time of exposure. 



Figure o. — Prophase of the first spermatocyte. Tlie chromatin tliread has broken into segments. Between 



these run delicate )inin fibers as at " a '', At " b "an element distinguishable from the others 



by its greater transparency and sharper outline. In these cells it is known as 



the accessory chromosome. Same object. 1000 diameters. 



There should now be visible a sharply defined, evenly illuminated image of the 

 object wherein the details are neither obliterated by excessive illumination, nor 

 rendered granular and obscure by deficient light. 



In obtaining a good negative, the manipulation of the substage iris diaphragm 

 is as important as the proper adjustment of the objective. No pains are spared, 

 accordingly, to bring about the appropriate arrangement of the cone of light, 

 and the lever of the diaphragm is swung back and forth until the very best 

 possible result is obtained. The Watson condenser above mentioned has a 

 graduation upon the mounting indicating the numerical aperture afforded by the 

 opening of the diaphragm. This is a convenience which should find a place 

 upon the condensers of other manufacturers. 



The further stages of the process are those which pertain to photographic 

 manipulations in general, and the limits of this article will not permit their con- 

 sideration. Summing up the steps to be followed, we have : 



