34 
Dr. Wollaston on the 
images, I have, in fig. 7, marked those surfaces which have 
their ccncavities toward the tube with large letters, A,B,C, 
and those curved in the opposite direction with the small let- 
ters, d , e,f ; by which means, in fig. 8, where the origin of each 
image is indicated, similarity of source in the several groups 
will be seen from similarity of notation. The concurrence 
of a pair of large letters, or a pair of small letters, as BC, or 
de, shows combination of curvatures in the same direction as 
in figures 3 and 4 ; unequal letters in the order d C, show 
opposite curvatures, as of fig. 5, giving a negative focus ; and 
the unequal letters, B /, in the opposite order, represent the 
opposite concavities of fig. 6, forming a positive focus beyond 
the glasses. 
The origin of these images was ascertained by giving a 
small motion to one or the other of the convex glasses. When 
the outer glass is inclined, fig. 9, all the images dependent 
on A or d are inclined together in the same line gh; but it 
may be remarked, that the image Ad is not displaced by this 
motion, as the relative position of the two surfaces A and d 
to each other remains unaltered. 
In fig. 10, the inner glass is represented with a similar 
inclination; and with it all images dependent on Cor/ for 
their formation, assume an inclined position in the line ik. 
By lateral motion of the outer glass, it is only the images 
dependent on A, that are moved ; for since the motion takes 
place in the direction of the curve d, which remains in con- 
tact with the middle glass, the position of this surface is not 
altered, fig. 11. 
By similar motion of the inner glass, fig. 12, those images 
only that are owing to/ are moved, while those from C retain 
their position. 
