ON THE MECHANISM OF IIHE ETE. 



57 



instrument to the use of presbyopic eyes, the 

 other end must be furnished with a lens of 

 four inches focal length; and a scale inpst 

 be made near the line on each side of it, di- 

 vided from one end into inches, and from 

 the other according to the table here calcu- 

 lated, by means of which, not only diverging, 

 but also parallel and converging rays from 

 the lens are referred to their virtual focus. 

 If ivory be employed, its surface must be left 

 without any polish, otherwise the regular re- 

 flection of light will create confusion ; and in 

 this respect, paper is much preferable. 



The instrument is easily appUcable to the 

 purpose of ascertaining the focal length of 

 spectacles required for myopic or presbyopic 

 eyes. Mr. Gary has been so good as to fur- 

 nish me with the numbers and focal lenu;lhs 

 of the glasses commonl}' made ; and I have 

 calculated 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, by 

 using the glasses of the focal length placed 

 opposite to the nearest crossing of the lines ; 

 and a myopic eye, with parallel rays, by 

 using the glasses indicated by the number 

 that stands opposite tlieir furthest crossing. 

 It cannot be expected, that every person, on 

 the first trial, will fix precisely upon that 

 power which best suits the defect of his sight. 

 Few can bring their eyes at pleasure to the 

 state of full action, or of perfect relaxation ; 

 and a power two or three degrees lower than 

 that which is thus ascertained, virill be found 

 sufficient for ordinary purposes. I have also 

 added to the second table, such numbers as 

 will point out the spectacles necessary for a 

 presbyopic eye, to see at twelve and at 

 eighteen inches respectively: tli« middle series 



will perhaps be the most 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 inter- 

 section may be accurately ascertained, by 

 means of an index sliding along the scale. 



The optometer is represented in Plate 9- 

 Fig. 72 and 73 ; and the manner in which 

 the lines appear, in Fig. 74. 



Table i. 



For extending the scale by a lens of 

 4 inches focus. 



412.00 



S 2.22 



2.40 

 2.55 

 2.671 

 2.77 

 2.88 

 2.93 

 3.00 



3.06 



3.11 



3.16 



3.33 



3.4 



3.52 



3.64 



3.70 



3.7 



70 



80 3.61 



100 



200 



00 

 — 200 

 — 100 4.17 

 — 50 4.35 

 — 45 4.39 



3.76 



3.85 

 3.92 

 4.00 

 4.08 



-40 4.44 

 -3S 4.51 

 -30 4.62 

 -25 4.76 

 -2o!5.00 

 -15^5.45 

 -14 5.60 

 -13|5.78 

 -I2|C.00i 



— 11 



— 10 



— 9.5 



— 9 



— 8.5 



— 8.0 



6.^9 

 6.67 

 6.90 

 7.20 



7.5s 

 S.OOi 



Table u. For placing the numbers indicating 

 the focal length of convex glasses. 



xvm. 



18.00 

 32. 7J 

 36.00 

 4.^.00 

 50.40 

 58.50 

 72.00 

 99.00 



180.00 



00 

 — 144.00 



— 63.00 

 36.00 



— 28.29 

 22.50 

 18.00 

 14.40 

 11.45 



9.00 

 5. 93 

 6.00 

 5.14 

 4.34 

 3.60 



VOL. II. 



4e 



