ON THE MECHANISM OF THE l'.\ K.
■67
instrument to the use of presbyopic eyes, theother end must be furnished with a lens offour inches focal length; and a scale mustbe made near the line on each side of it, di-vided from one end into inches, and fromthe other according to the table here calcu-lated, by means of which, not only diverging,but also parallel and converging rays fromthe lens are referred to their virtual focus.■- If ivory be employed, its surface must be leftwithout any polish, otherwise the regular re-1 flection of light will create confusion; and inthis respect, paper is much preferable.
The instrument is easily applicable to thepurpose of ascertaining the focal length ofspectacles required for myopic or presbyopic. eyes. Mr. Cary has been so good as to fur-nish me with the numbers and focal lengthsof the glasses commonly made ; and 1 havecalculated the distances at which those num-bers must be placed on the scale of the opto-meter, so that a presbyopic eye. may be en-abled to see at eight inches distance, byusingi the glasses of the focal length placedopposite to the. nearest crossing of the lines;and a myopic eye, with parallel, rays, byusing the glasses indicated by; the numberthat stands opposite their furthest crossing.It cannot be expected, that every person, onthe first trial, will, fix precisely! upon thatpower which best suits the defect of his sight.Tew can bring their eyes at pleasure to thestate of full action, or of perfect relaxation ;and a,power two or three degrees lower thanthat which is thus ascertained, will be foundsufficient for ordinary purposes. 1 have alsoadded to the second table, such numbers tvswill point out the spectacles necessary for apresbyopic eye, to see at twelve and ateighteen inches respectively; the middle series
will perhaps be the most proper for placing-the numbers on the acute. The optometershould be applied to each eye; and, at thetime of observing, the opposite eye shouldnot be shut, but the instrument should bescreened from its view. The place of inter-section may be accurately ascertained, bymeans of an index sliding along the scale.
The optometer is represented in Plate <)•Fig. 72 and 73; and the manner in whichthe lines appear, in Fig. 74.
Table i. For extending the scale by a lens of• 4 inches focus.
<
3.00
13
3.06
70
3.78
.— 40
4<44
—11
6>20
5
3.32
14
3.11
HO
3.81
.— 35
4.51
— 10 .
6.67
6
3.40
15
3.ie
100
3.85
j —30
4.62
— 9.5
6<00
7
2.55
20
3.38
$00
3.92
1—25
4.70
— 00
7.20
8
2.67
25
3.45
00
4.00
20
5.00
— 8.5
7-56
9
2.77
30
3.52
—200
4.08
— 15
5-45
— 8.0
8.00
10
2.86
40
3.64
1—100
4.J7
—14
5.60
n
2.03
50
3.?0
4.35
—) 3
5.78
is
3.00
60
3.75
j —45
4.39
—12
6.00
t
' Tabic n. For placing the numbers indicatingthe focal length of convey glasses.
Foe .
VIII.
xir .
• XVIII.
00
8.00
14.00
18.00
40
10.00
1744
32.73
36
10.28
18.00
36.00
sot
10.91
2 O .00
45.00
28
• 11.20
21.00
.50*40
26
11.56
22.20
58.50
24
12.00
24.00
.72-00
# 22
12.7>
26.40
99 00
*20
13.33
30.00
180.00
18
14.40
36,00
00
16
16^00
48.00
—144.00
14
18.67
.,84.00
—* 63-00
12
24.00
00
— 36-00
Ul
20.33
—132-00
— 28-20
10
4,0.00
— 60.00
—r 22.50
9
72.00
— 36.00
— 18.00
8
00 ■
— 24.00
— 14.40
7
—56.00
— 16.80
— 11,45
6
—24.00
— 12.00
— 9.00
5
— 13.33
— 8.57
— 5(92
4.5
—10.20
— 7-20
— 6.00
4.0
8.00
— 6.00
— 5.14
3.5
— 6.22
— 4.94
— 4.34
3.0
—* 4.80
—* 4.00
1
— 8.6®