Dr. Wollaston on a triple object-glass. 33 
combinations of these surfaces is fifteen; and just so many 
images formed by reflection may be discerned. 
It is manifest, that if the glasses be duly adjusted to each 
other, so that their axes are correctly coincident, then this 
series of images must be all situated in the same straight 
line; and conversely, that any defective position may be im- 
mediately detected by a derangement of the line of images. 
A very distinct view of a part of these appearances, fig. 1. 
(pi. 1.) is seen by placing the eye close to the object end of 
the telescope, so as to view the eye-hole illuminated by the 
flame of a candle set near it. In this position only ten of 
them are perceived; two beyond the refracted image; four 
in regular succession nearer to the eye ; and four very small 
ones close together, at a short distance from the object-glass 
within the tube. 
The remaining five images being some inches exterior to 
the object-glass, cannot be seen till the eye is withdrawn to 
a greater distance, and are best observed with the assistance 
of a lens, fig. 2. 
Each of the two first named images is formed by a pair of 
surfaces that are curved in the same direction, fig. 3, of 
which that which reflects from its concave side is most curved. 
The next four are also formed by surfaces that are curved in 
the same direction ; but in this case, the convex reflecting 
surface is more curved than the concave, fig. 4. The four 
small images arise from pairs of surfaces, that have their 
convexities opposed to each other, fig. 5 ; and the five ex- 
terior images are owing to those pairs of surfaces which pre- 
sent their concave sides to each other. 
In order to explain distinctly the origin of each of these 
mdcccxxii. F 
