8 14 DA maskelyne’s Account of 
focus of tile eye-glafs, and to be of fuch a fize as to cover 
the aperture of the objed-glafs when brought dole to it. 
By the refraction of thefe wedges two images were 
formed in the telefoope, which were at the greateft dif- 
tance (about 36') when the wedges were dole to the ob- 
jed-glafs, and approached as they were moved towards 
its focus, where they united; lo that the whole focal 
diftance of the object-glals was to be the length of the 
fcale for meafuring the angular diftance of the two 
images formed in the telefcope. When thefe wedges 
were applied, as above defcribed, the two images were 
found to be coloured to a great degree, occafioned by the 
refradion of the wedges. This defed you di reded me 
to remove by making the prifmatic glaffes or wedges 
achromatic, on the fame principles as the achromatic 
objed-glafles ; and, after fome difficulties, this was ef- 
feded ; the two images formed in the telefcope appeared 
free from colours and diftind. The above experiments 
were made in a rough wooden tube, with an inconve- 
nient method of moving the wedges by hand : in this 
ftate it was when fliewn to Alexander aubert, efq. 
F. r. s. towards the end of May, 1776 ; after which you 
deftred to have it done in a more compleat manner, in a 
brafs tube, with a means of turning the tube round to 
take angles in different diredions, and a method of 
moving 
