170 PROCEEDINGS OF THE AMERICAN ACADEMY 



is represented in Fig. 2. Tlie two small lens- 

 es, L', having with their united forces a focal 

 length of less than an inch, receive the converg- 

 ing rays as they come from the large lens, L, 

 and make them still more convergent. It is 

 necessary that the quartz should be adjusted 

 very near the focus of the rays, and also that the analyzer should be 

 placed as closely as possible in front of the quartz. Otherwise, this 

 highly divergent pencil of light, after it leaves the focus, will not be 

 able to get through the small area of the crystal and analyzer, and 

 therefore the whole field of view will be curtailed. The crystal and 

 analyzer are held by separate rings, which are supported on uprights 

 which slide independently upon a horizontal bar, and the two rings 

 may, therefore, be pushed as closely together as the thickness of the 

 crystals will allow. 



" In Soleil's apparatus, there is an opening, such as is seen at O of 

 Fig. 1, similar to that made in magic-lanterns, and for the same object, 

 namely, the introduction of the frame containing the object to be 

 magnified. When Soleil's apparatus is used for the inspection of 

 large objects, such as pieces of tempered glass or artificial selenite 

 figures, they are introduced at this opening, and in these cases the 

 large lens serves as a magnifying-glass, and there is no condensing 

 lens to help the illumination. This defect would not be felt with very 

 large objects, as large, that is, as the lens itself; because the object 

 itself would receive immediately as much light as the lens could 

 throw upon it. But the objects to be examined are seldom, if ever, 

 of this magnitude. Hence, much of the light which might otherwise 

 be saved, is lost for want of a condensing lens. In order that the ap- 

 paratus may retain all the advantages which belong to it in its original 

 form, I have retained the opening 0, which may still be used in any 

 cases where it is prefei'red. But the tube which carries the large lens 

 has been divided at the middle of its length, and one part made to 

 slide in the other. Another opening, O', similar to that in the original 

 apparatus has been joined to one end of the interior tube. By push- 

 ing more or less of this tube into the next exterior one, the objects 

 placed in the new opening can be put at smaller or larger distances 

 from the principal lens, which is to this opening an illuminating 

 lens. The best distance, in any given case, is that which makes the 

 section of the cone of rays by the object equal to that part of its own 



