8lO Dr. MASKELYNE’s AcCOUtlt of 
diftance of the prifms from the principal focus is the 
fame as it was found from the actual focus in the ter- 
reftrial experiment, as the principal focal diftance of the 
objedt-glafs is to the actual focal diftance in the laid ex- 
periment. 
It will, I apprehend, he the beft way in practice, in- 
ftead of one prifm to ufe two prifms, refradting contrary 
ways, and fo divide the refradtion between them (as re- 
prefented in fig. 3. and 4.). Achromatic prifms, each 
compofed of two prifms of flint and crown-glafs, placed 
with their refradting angles contrary ways, will un- 
doubtedly be neceffary for meafuring angles with great 
precifion by this inftrument : and I can add with plea- 
fure, that I find by experiment made w r ith this inftru- 
ment, as it was executed by Mr. dollond with achro- 
matic prifms, ground with great care for this trial above 
a twelve-month ago, that the images after refradtion 
through the prifms appear very diftindl; and that obfer- 
vations of the apparent diameters of objedts may be 
taken in the manner here propofed with eafe and pre- 
cifion. 
Two or more fets of prifms may be adapted to the 
fame telefcope, to be ufed each in their turn, for the more 
commodious meafurement of different angles. Thus it 
may be very convenient to ufe one fet of prifms for mea- 
1 furing 
