5 
of Achromatic Object- Glasses . 
The sights must now be placed as shewn in Plate I. Fig. 1. viz. 
the one towards the edge of the prism, above the zero or horizon- 
tal line, and the one towards the base below the same, and the 
lower the better, setting it to some certain reading, as, for ex- 
ample, 60° or 55°, &c. 
Place on the table, under the lower sight, a piece of clean 
white paper, and reflect upon it (if necessary) a strong light ; 
bright sunshine is to be preferred. Then move about the upper 
sight till the eye perceives the refracted image of the lower sight 
bisected by the straight edge, and note its reading. These are all 
the data requisite for commencing the calculation ; but, for the 
sake of greater security, it will be best to repeat the observation 
under three or four different incident angles. 
The image seen in this experiment will be coloured and elon- 
gated, but there will still, with a little practice, be no difficulty 
in bisecting it. 
Note . — In order to prevent any confusion in the computation 
arising out of the signs of the cosines above and below 90°, 
it will be best to register the supplements of the actual read- 
ings, or what they want of 180°, instead of the readings 
themselves. 
6. The following are a set <f Observations on the above prisms* 
Flint Prism , No. 1. 
Supplement to 
Supplement to 
Half the 
Angle of 
No. 
reading of 
reading of 
AJL till lllV^ , 
differen ce. 
Prism as 
Lower Index. 
Upper Index. 
above found. 
(Q.) 
(P-) 
\ a ') 
(a.) 
1 
120° O' 
104° 10' 
7° 55'- ) 
2 
125 0 
108 45 
8 7 ( 
3 
130 0 
113 0 
8 30 ( 
24 49' 
4 
135 0 
117 10 
8 55 J 
Plate Prism 
, No. 1. 
Supplement to 
Supplement to 
TTolf ffrp 
Angle of 
No. 
reading of 
reading of . 
•Ex diJL lUc 
diffprpnpp 
Prism as 
Lower Index. 
Upper Index. 
UllivJlvUCv* 
above found. 
1 
120° O' 
106° 0' 
7° 0' 
2 
125 0 
110 38 
7 11 ( 
3 
130 0 
115 0 
7 30 r 
24° 51' 
4 
135 0 
119 40 
7 40 ) 
