r.f Double Stars, ^q 



HI. loZ. Fl. i^'^ AquiliE pra?cedens ad boream. 

 July 7, Double. Above | n. preceding the ipth, in a line 

 1783, parallel to jG and ^ Aqullae. Very unequal. L. r. ; 

 S. dr. Diftance 12^' 58^^^ PoUtion 58^ 27^ f. fol- 

 lowing. 

 109. Fl. ip^'^Aquilae praecedens ad Boream. 

 July 7, Double. About If degree n. preceding the 19th, in 

 1783. a line parallel toe and (J'Aquihr. Pretty unequal. Both 

 rw. Diftance lo'" 13'''. Poiition 22'' 6^ n. preceding. 

 no. Fl. 77^ Cygni borealior et prcTcedens. 

 Sept. 17, Quadruple. Full | degree n. preceding the 17th, in 

 1783. a line parallel to a- and « Cygni ; a fmall ftar. The 

 two nearefl: extremely unequal. L. r. ; S. d. Diftance 

 with 625, 13''' 54'''. Politlon 6f 36' f. following. 

 The two largeft a very little unequal. Both r. Dif- 

 tance with 278, 25'' 58''". Pofitioa 40'' 33^ n. fol- 



and low obfervations, is evident from fome meafures I have taken to afccrtain its 

 quantity. Thus I found, May 4, 1783, that the perpendicular diameter of e, 

 Flamsteed's 20thSagittarii, meafured 16" 9' ', while the horizontal was 8"35'" ; 

 which gives ']" 34'" for the prifmatic effeft : the meafures were taken with 460, 

 near the meridian, and the air remarkably clear. And though this power, which 

 depends on the obliquity of the incident ray, diminifhes very fall in greater alti- 

 tudes, yet I have found its effcfts perceivable as high, not only as « or y Corvi in 

 the meridian, but up to Spica Virginia, and even to Regulus. Experiments on 

 thefe two latter flars I made November 20, 1782 ; when Regulus, at the altitude 

 of 4g°, fliewed the purple rather fuller at the bottom of the field of view than 

 when it was at the upper tdge ; which fliews that the prifmatic powers of the edges 

 of the eye lens were affifted in one lituation by the power of the atmofphere, but 

 counterafted by it in the other. I turned the eye lens in all fituations, to convince 

 myfelf that it was not in fault. This experiment explains alio, why a ftar is nor, 

 always befl in the center of the field of view ; a fad I have often noticed before I 

 knew the caufe. 



Vol, LXXV. N lowing. 



