VOL, 



LXXII.] 



PHILOSOPHICAL TRANSACTIONS. 



235 



which was thrown on a sheet of paper exactly 8± inches from the lens, the eye 

 being always as close to the lens as possible. I viewed the same wire, exactly in 

 the same manner, with every one of the lenses, and measured the pictures on 

 the paper. When I came to the higher powers, the wire was exchanged for 

 another, 4.37 times thinner than the former, as determined by comparing the 

 proportion of their images 54 to 235-2-, taken by the same lens. 



When the images of these wires are obtained, the power of the telescope, 

 with every one of the lenses, becomes known by one plain analogy : viz. as the 

 image of the wire by the first lens (77\) is to the power it gives to the telescope 

 (l63.8,) so is the image of the wire by the 2d lens (119,) to the power it will 

 give to the same telescope (250.7.) The particulars of all the measures are as 

 follow : 



Powers as they 

 have been called 

 in my papers. 



146 



Images of a wire thrown A mean Powers as they 



on a paper in hundredths of of the 4 come out by this 

 half inches. measures. method. 



77.. 



78 . . 78 11% 16'3.S6 = 



170.4 

 1.0+ 



{;: 



.... 119.. 119.. 119 . 



143.. 143 .. 144 . 



236. . 236' . . 235 . . 



.... Smaller wire. 



53.. 54.. 55.. 



83. . 85 . . 84 .. 



107.. 107 .. 107 .. 



128.. 128 .. 129.. 



1536' An excellent lens, 



2010 236. . 236 . . 238 . 



3168 281.. 283 ..281 



6450 635. . 6".'5 . . 630 . 



227 

 27S 



460 



754 



932 



1159 



119 



143^ 



235| 



119.- 

 143 .. 



236 . . 



. 54 . . 

 . 85 .. 

 ,108.. 



128 . . 

 lost about S months before, 



236' 236 



280 281 



626 629 



54 



8+J 



107J 



P' 



250.7 

 ,301.8 



496.7 



.775.1 



, 986.7 

 H79 9 



2175.8 



2585.5 

 57 86.8 



I beg leave, Sir, now to give a short description of the method I have formerly 

 used to determine these powers. In the year 1776 I erected a mark of white 

 paper, exactly half an inch in diameter, which I viewed with my telescope at the 

 greatest convenient distance with one of the least magnifiers. An assistant was 

 placed at right angles in a field, at the same distance from my eye as the object 

 from the great speculum of the telescope. On a pole erected there I viewed the 

 magnified image of the half inch, and the assistant marked it by my direction; 

 this being measured, gave the power of the instrument at once. The power 

 thus obtained was corrected by theory, to reduce it to what it would be on infi- 

 nitely distant objects. The powers of the rest of the lenses I deduced from this, 

 by a camera eye-piece, which I made for that purpose, abcd (fig. 17, pi. 3) re- 

 presents a perpendicular section of it. The end a screws into the telescope. 

 On the end b may be screwed any of the common single lens eye-pieces. Imn 

 is a small oval plane speculum, adjusted to an angle of 45° by 3 screws, 2 of 

 which appear at o, p. When the observer looks in at b, he may see the object 

 projected on a sheet of paper on a table placed under the camera piece, and mea- 



H H 2 



