Prof, 0. N. Rood on the Investigation of Microscopic Forms. 211 
XXIV.—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 disk 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. 
The 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 firmly convinced 
that this method of determination is destined hereafter to play a 
most important part in microscopical observation. To the mi- 
croscopist it will prove as powerful a means of investigation as 
it now is in the hands of the optician. 
_ The most convenient and effective mode of proceeding in this 
ease which has occurred to me is the following :—The microscope 
is brought into a horizontal position, the mirror removed, and the 
illumination supplied by a candle or lamp placed in the axis of 
the compound body at a distance of not more than three inches 
from the stage. If now a small sphere of glass be properly sup- 
ported on the stage, it forms behind itself a very minute inverted 
image of the flame of the candle; upon drawing back the com- 
pound body slightly, this image comes into focus, and is seen of 
course in an erect position. When a rod of ;4,th of an inch in 
thickness is moved up and down between the flame and the glo- 
bule, an image of it is seen in the microscope with great distinct- 
ness, and it is observed that the motion of this image follows in 
all respects the motion of the hand. Upon replacing the sphere 
by a minute concave lens, as an air-bubble in water, the reverse 
takes place ; to gain distinct vision of the flame, it becomes neces- 
sary to move the compound body within the focus; the image of 
the flame is seen to be inverted, and, what is practically more im- 
portant, the motion of the rod seems reversed. It will happen 
very generally, in applying this method, that the image of the 
flame is not sufficiently perfect to decide whether it is erect or 
inverted ; the motion of the rod then furnishes us with a certain 
means of deciding this point : if its motion is natural, the image 
is erect, and the curvature convex, &c. After some practice, it 
becomes easy to obtain the best focal adjustment for distinct 
vision of the rod, and in extreme cases where the image is very 
* From Silliman’s ‘ American Journal of Science,’ &e., Jan. 1862, 
