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O. N. Rood on the investigation of Microscopic forms. 65 
Art. VIII.—On the investigation of Microscopic forms by means 
of the Images which they furnish of external objects, with some 
practical applications ; by Prof. O. N. Roop of ‘Troy, N. Y. 
Ir would hardly occur to a physicist, who was requested to 
determine whether a certain disc of glass was a convex or a 
concave lens of slight curvature, to attempt a solution of the 
question by glancing along the two sides; on the other hand, 
neglecting even to look at the glass, he would at once bestow 
his undivided attention upon the images of external objects 
formed by it, and thus with ease and certainty decide upon the 
nature, degree, and regularity of its curvature. 
he simple idea here enunciated seems hardly to have been 
applied to the study of microscopic forms, though from some 
experiments lately made in this direction, I am fi on- 
vinced that this method of determination is destined hereafter to 
play a most important part in microscopic observation. To the 
microscopist it will prove as powerful a means of investigation 
as 1t now is in the hands of the optician. 
€ most convenient and effective mode of proceeding in this 
case which has occurred to me is the following: the microscope 
is brought into a horizontal position, the mirror removed, and the 
umination supplied by a candle or lamp placed in the axis of 
the compound body at a distance of not more than three inches 
m the stage. If now a small sphere of glass be properly sup- 
ported on the stage, it forms behind itself a very minute inve 
mage of the flame of the candle: upon drawing back the com- 
pound body slightly this image comes into focus and is seen of 
a>. generally in applying this method that the image of the 
ime 1s not sufficiently perfect to decide whether it is erect or 
