MEMORANDA. 



Ill 



placed the object to be examined. The rays are brought to 

 a focus (f), at some distance above the object (o), thus giving 

 an even white light over the whole field of vision, and this I 

 have found particularly advantageous when using low powers 



ri>. 2. 



for viewing objects. We can modify this arrangement by 

 placing the lens below the stage (as in fig. 2), and at a point 

 where it can be adjusted by a rack and pinion, so that the 

 rays from it are focussed on the object. HoAvever, the first 

 arrangement I have found the most generally useful. To 

 this can be added a diaphragm (fig. 3), which any one can 

 make for himself, to fit the microscope, of blackened card- 

 board. The general utility and cheapness of this simple 

 contrivance will, I think, recommend it to the considera- 

 tion of all whose purses are not as large as their desire for 

 knowledge. 



I should also mention that I have found this illuminator 

 particidarly useful on dull days, when, by the mirror alone, 

 only a gray light could he oljtained, while, with the addi- 

 tion of my condensing lens a clear white light is obtained ; 

 and, from the reason of most of the rays falling obliquely on 

 the object, the markings of certain of the Diatomacese can 

 be easily resolved with it. When the diaphragm (fig. 3) is 



