of the Angle fubtended by Two Objefls, &c. 4^3 
The divarication of the images while they traverfe the field 
of the telefcope during the time of -an obfervation, and the 
errors of the obferved angle in confequence of any change in 
the quantities m * and n (p. 429 ) ffiould they appear to be of 
fenfible magnitude, may be diminifhed until they are in 
phy ileal fenfe evanefeent, by altering the values of s and m 
(art, 51.) : for this purpofe it will be requifite to life two fe- 
parate conilruclions ; the one for obferving very final 1 angles, 
thofe, for example, which do not exceed 2 7 : and the other,, 
for meafuring fuch angles as are fubtended by the fun and moon,. 
In the former of thefe j may be afliimed rr the fine of 30",. 
m — fin. p°, and n — o. From thefe data tab. in. is calcu- 
lated. In the other conftrudtion, becaufe very fmall reflecting 
furfaces are neceffary to obferve the fun by two reflections (art. 
ip.) m may be afliimed ~ fin. i°, s being — fin. iofl and 
n — o : from which conditions the fourth table is calculated. 
Thefe tables may be eafily extended, by calculating from the 
value of cof. ED or fin. |ED 2 contained in art. 21. or 22. 
* Let the telefcope with the entire conflrudion be fieadily fixed : then if the 
objeds obferved be in contad while the touching points occupy the center of the 
field, the angle of incidence on the fixed fpeculum will be of its true magnitude ; 
that is, its fine or m will — fin. 7 0 in tables I. IT. and III.; and m — fin. i Q in 
table IV. The fixed plane of refledion alfo will be coincident with the primitive 
fecondary in all the conftrudions correfponding to tables I. II.. III. and IV. The 
telefcope continuing unmoved, the diurnal motion of the heavens will' caufe the 
points in contad to leave the center of the field ; this willoccafion no alteration in 
the quantities j and p , but will afled m and n only : therefore, I and p — o. To 
eftimate the effeds of this change in the values of m and », let ?n~ c x A i—nf, 
and nzzdx^i — n~ , as in art. 28. and fuppofe a line to be drawn through the centre 
of the field in a plane, perpendicular to the plane of motion : this line will be in the 
fixed plane of refledion ; and any deviation of the points in contad through a 
fmall angular, ip ace c, from the center of the field will caufe an equal variation c im 
the-.'. 
