£ 97 ] 
at right angles to the index glafs, and confequently 
will be properly adjufted for ule, and the obfer- 
vations may be made with the fame accuracy by 
this, as by the fore obfervation. 
To adjuft the horizon glaffes in the perpendicular 
pofition to the plane of the inftrument, I have con- 
trived to move each of them by a fingle fcrew, that 
goes through the frame of the quadrant, and is turned 
by means of a milled head at the back, which may 
be done by the obferver while he is looking at the 
object. 
To thefe improvements, Sir, I have added your 
method of placing darkening glaffes behind the ho- 
rizon glaffes, which yGU have been fo kind as to give 
me liberty to apply to my inftruments. Thefe 
glaffes, which ferve for darkening the objeCt feen by 
direct vifton, in adjufting the inftrument by the Sun 
or Moon, I have placed in fuch a manner as to be 
turned behind the fore horizon glafs, or behind the 
back horizon glafs, that they may be ufed with 
either; there are three of thefe glaffes of different 
degrees of darknefs ; the lighted; or paleft: I do 
imagine will be of ufe in taking the Sun’s altitude 
when the horizon appears glaring, which I believe 
often happens by the reflection of the fea. 
If thefe additions and alterations fhould be thought 
to be real improvements, which I cannot doubt. Sir, 
if they are honoured with your approbation, I hope 
they may ferve in conjunction with thofe improve- 
ments you have made yourfelf in refpeCt to the ob- 
viating any poflible errors in the parallelifm of the 
planes of the index glafs, and in regard to the ad- 
juftment of the telefcope parallel to the plane of the 
Vql. LXII. O quadrant* 
