37 
on the Mechanism of the Eye . 
proper for placing the numbers on the scale. The optometer 
should be applied to each eye ; and, at the time of observing, the 
opposite eye should not be shut, but the instrument should be 
screened from its view. The place of intersection may be accu- 
rately ascertained, by means of an index sliding along the scale. 
The optometer is represented in Plate III. Fig. 8 and g; and 
the manner in which the lines appear, in Fig. 10. 
Table I. For extending the scale by a lens of 4 inches focus. 
4 
2.00 
1 1 
2-93 
30 
3 - 5 2 
200 
3 - 9 2 
-35 
4.51 
— 12 
6.00 
5 
2.22 
12 
3.00 
40 
3- 6 4 
00 
4 00 
-30 
4.62 
— 11 
6 29 
6 
2.40 
>3 
3,06 
5 ° 
37 ° 
— 200 
4.08 
-25 
4 76 
— 10 
6.67 
7 
2 -S 5 
H 
3 ” 
60 
375 
— 100 
4 17 
— 20 
5.00 
-9 5 
6.90 
8 
2.67 
15 
3 16 
70 
378 
- 5 ° 
4 35 
-15 
5-45 
- 9 *o 
7. 20 
9 
2 77 
20 
3 33 
80 
3.81 
— 4 ? 
4 39 
— H 
5.60 
-8.5 
7.56 
10 
2.86 
2 5 
3-45 
100 
3-85 
-40 
4.44 
“i 3 
578 
-8.0 
8.00 
Table II. For placing the numbers indicating the focal length of 
convex glasses. 
1 
Foe. 
1 
VIII. 
XII 
XVIII. 
Foe. 
VIII. 
*„. 
| 
XVIII. 
Foe. 
VIII. 
XII. 
XVIII. 
0 
8.00 
12 OO 
I8.CO 
20 
1 3 • 3 3 
30.00 
1.80.00 
8 
00. 
— 24,00 
-14.40 
4 ° 
10.00 
l I 7- I 4 
32 73 
18 
I 4 - 4 ° 
36.00 
00 
7 
—56.00 
— 16 80 
-11.45 
36 
1© 28 
18.00 
36,00 
16- 
16 00 
48.00 
— 144.00 
6 
— 24.00 
— 12.00 
— 9.00 
3 ° 
10.91 
[20.00 
45.00 
1.4 
18.67 
84.00 
— 63.00 
5 
1 1 3 -3 3 
- 8 57 
- 5.92 
28 
11 20 
|21.00 
50.40 
12 
24.00 
00 
— 36.00 
4-5 
— 10.29 
— 7.20 
— 6.00 
26 
11.56 
22. 29 
58 50 
1 1 
2 9-33 
— 132.00 
28,29 
4.0 
— 8.00 
— 6.00 
- 5-14 
24 
12,00 
[24 00 
72.00 
10 
40.00 
— 60.00 
— 22.50 
3-5 
— 6.22 
- 4.94 
- 4-34 
22 
1277; 
26 40 
99.00 
9 , 
72.00 
— 36.00 
— 18.00 
3 -o 1 
4.80 
— 4.00 
- 3-6 
Table III. For concave glasses. 
Number. 
Focus and 
furthest 
place. 
Nearest 
place. 
Number. 
Focus and 
furthest 
place.. 
Nearest 
place. 
Number. 
Focus and 
furthest 
place. 
Nearest 
place. 
0 
4.OO 
; 7 
8 
2.67 
14 
3-00 
I.71 
1 
24 . 
3*43 
8 
7 
2-54 
15 
2.75 
1.63 
2 
18 
377 
9 . 
6 
2.40 
1.6 
2.5O 
I.54 
3 
16 
3.20 
10 
5 
2.22 
1 7 
225 
i -44 
4 
12 
3.00 
11 
' 4*5 
2.12 
18 
2.00 
i -33 
5 
IO 
2.86 
12 
4.0 
2.00 
19 
175 
1.22 
6 
9 
2.77 
13 
3-5 
1-87 
20 
1.50 
1. 02. 
