ef I)sM$k Starts " '■ 



III. loS. Fn. r9»'» Aqufepra^cedens ad boream* 

 July 7, Double. Above | n. preceding the iptli^ in a \m^ 

 1783. parallel to jG and f Aquili£. Very unequal. L. r. ;, 

 ^ S. dr. Diftance 12^' 58^"^ Pofition 58^ 27'' f. fol« 



lowing. 

 109. Fl. 19*"* Aqurlas priEcedens ad Boream» 

 July 7, Double. About ij degree 11. preceding the 19th, iur' 

 1783. aline parallel toe and ^i\quiliE. Pretty unequal. Both 

 rw. Diftance 10^^ I3^^^» • Portion 22'^ 6^ n. preceding. 

 no. Fl. 77' Cygni borcalior et pr^ecedens. 

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

 1783. a line parallel too- and ^j^ Cygni ; a fmall ftar. The 

 two neareft extremely unequal. JL. r. ; S. d, Dhlance 

 with 625, 13'' 54^''. Poiition 67" 36^ f. following. 

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

 tance with 278, 25^' S^''- t'oiitioii 40-^ 35'' i^- ^^l' 



and low obfervations, is evident from forae meafurcs I have taken to afceitain its 

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

 Flamsteed's aoth Sagittarii, roeafured 16" 9' ', while the horizontal was 8"35'" ; 

 which gives 7" 34""' for the prlfmatic effeft : the meafures were taken with 460,. 

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

 depends on the obliquity of the incident ray, diminiflies very faft in greater alti- 

 tudes, yet I have found its efFcds perceivable as high, not only as a or 7 Corvi in 

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

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

 of 49", fliewed the purple rather fuller at the bottom of the field of view than 

 wheii it was at the upper edge ; which fliews that the prifmatic powers of the edges 

 of the eye lens were affifted in one fituation by the power of the atmofphere, bu.t 

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

 myfelf that it was not in fault. This experiment explains alio, why a liar is not 

 always beft in the center of the Held of view ; a fact 1 have often noticed before I 

 knew the caufe. 



V0L. LXXV. N lo^Ying. 



