concentric adjustment of a triple object-glass. 35 
Any lateral motion given to the central concave has the 
same effect as moving both the convex glasses together in the 
opposite direction, and of course the effect is most discernible 
in the position of an image that is similarly affected by the 
lateral motion of both. For instance; downward motion of 
the concave, fig. 13, has the effect of placing a wedge between 
the two convex glasses from above, so that though each of 
their inner surfaces still remains in contact with the adjacent 
concave, their exterior surfaces, A and f, are parted from 
above, and the image Af, dependent on them both, is doubly 
elevated. Hence, the position of Af becomes a delicate test 
of the due centering of the concave glass, and is the best 
guide in making the final adjustment. In my object-glass, 
which has been the principal subject of my experiments, this 
image is very happily situated for this purpose, being so near 
to the outmost image, Ad, that the smallest error in their 
relative position is with the greatest facility detected. 
In order that I might have full command of each part, I 
had a cell constructed of larger dimensions than ordinary, 
with two pair of adjusting screws, at right angles to each 
other, applied to the edges of each glass, so that when the 
images had been first brought into the same vertical plane by 
means of one set of screws opposed to each other in the ho- 
rizontal position, the series might next be adjusted to the same 
horizontal line by the screws placed above and below, at right 
angles to the former. 
In performing these adjustments, there is another pair of 
images beside Af, Ad, which deserve particular attention, as 
their motions are independent, and their contiguity renders 
any variation of their relative position very perceptible. In 
