BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 529 



Herschel. W.: Synopsis of the Writings of— Continued. 

 A. D. Vol. P. 



1782 72 163 Description of a Lamp Micrometer, and the method of using it. By Mr. 

 William Herschel, F. R. S. Read January 31, 1782. 



163 The imperfections of the parallel wire micrometer in taking the meas- 



ures of close double stars are [described] : 1st, that the measures 

 must include the diamters of the stars ; 2d, the deflection of the light 

 on the wires [single threads of the silk-worm were used] ; 3d, the 

 uncertainty of the zero ; 4th, the imperfection of the screw ; 5th, 

 the necessary illumination of the field. 



164 The Lamp-Micrometer is free from all these defects. Description and 



reference to Plate V. 

 169 The powers of Herschel's telescopes were [at this time] determined 

 by looking at a scale with one eye free and the other at the instru- 

 ment. 



169 Description of the method of observing. 



170 "A little practice in this business soon makes it easy, especially to one 



who has already been used to look with both eyes open." 

 Example of the measure of the position of a Herculis. 

 ("My telescope bears a power of 460 so well that for a twelvemonth 



past I have hardly used any other.") 



171 "A power of 932 on fine evenings is very distinct," and gives more 



than half an inch to a second on the Lamp-Micrometer. 



171 Other apiilications of this micrometer are to the measures of the di- 

 ameters of the planets and satellites, the mountains of the moon, 

 the diameters of the fixed stars, etc. 



171 Example of the measure of the diameter of a Lyrw [diameter 0".355]. 



171 Nov. 28, 1781. I measured the diameter of the new star [ Uranu8'\, 

 The diameter of this "singular star" was 5". 022. 



1782 72 173 A paper to obviate some doubts concerning the great magnifying powers 

 used. By Mr. Herschel, F. R. S. 

 [In the form of a letter to Sir Joseph Banks, Prest. R. S.] 



173 I have the honor of laying before you the result of a set of measures 



I have taken to ascertain once more the powers of my Newtonian 

 7-feet reflector. 



174 The method described. The solar focus of one of the eye-pieces was 



measured five times and found to be 1.01, 1.04, 1.09, 1.01, 1.05 in 

 half-inch measure. The sidereal focus of my 7-feet speculum in the 

 same measure is 170.4. The mag. power of that lens is, then, 163.8. 

 This eye-lens was then compared with others by measures of the 

 diameter of a brass wire [details of the experiment given]. 



175 Powers as they have been called in my papers, 146, 227, 278, 460, 754, 



932, 1159, 1536 [this lens lost 8 months ago], 2010, 3168, 6450. 

 Powers as they come out by this method, 163.86, 250.7, 301.8, 496.7, 

 775.1, 986.7, 1179.9, , 2175.8, 2585.5, 5786.8. 



175 Description of the method formerly used to determine these powers : In 



1776 a mark of white paper exactly half an inch in diameter was 

 viewed at the greatest convenient distance with the least magnifiers. 

 An assistant was placed at rectangles in a field at the same distance^ 

 and u])on a pole there erected I viewed the magnified image of the 

 half inch which was marked by my direction. The power thus ob- 

 tained was corrected to reduce it to what it would be upon infinitely 



176 distant objects. The powers of the rest of the lenses I deduced 

 from this bv a camera eye-piece. [See Plate VI, figs. 1 and 2.] 



S. Mis. 31 34 



