34 On the UNEQUAL 



beyond all doubt, that the colour obferved might not proceed 

 from the angles of the prifms not being precifely thofe, which 

 would render the correction of colour mod perfect, I paid no 

 farther attention to the fuhject at that time. 



In examining the object- glafs above mentioned, the object 

 obferved was a fmall window in a white wall, at the diftance of 

 feveral hundred yards to the eaftward of my ftation, the fun 

 ihining upon the wall from the weft. The circumftances of 

 the phenomena, which I have extracted from memorandums 

 written at the time of making the experiments, were as follow : 



" July 28. 1787. In conftruction A, (by this is meant the 

 ten inch object- glafs above defcribed), when rendered as achro- 

 matic as poffible, a purplilh light appears on one fide the focus, 

 and a greenifh light on the other." 



In the next obfervation of this kind of incorrigible colour, 

 the flame of Argand's lamp was ufed as an object, the great 

 brilliancy of its light rendering the phenomena more confpi- 

 cuous. A cylinder of brafs was placed over the glafs-tube, 

 which intercepted all the light, excepting what pafled through 

 a fmall round hole oppofite to the flame. I found no object 

 preferable to this for the purpofe, except the planet Venus, which 

 cannot always be commanded. My obfervation mentions, that 

 cl with the patent lamp, the colour is deep carmine within the 

 focus, and greenifh yellow without it." 



Another memorandum on this fubject runs thus : " Con- 

 ftruction 10. (by this is meant another object-glafs, compofed, 

 like the former, of crown-glafs, an effential oil, and fpirit of 

 wine, inftead of aether, but a few inches longer than it, and 

 more perfect) difcovers a great deal of colour of fome kind, 

 in covering half the object glafs. The object, though coloured, 

 is then more diftinct than upon uncovering the other half; the 

 colour is thus converted into miftinefs. On altering the difper- 

 fion of the fluid, the colour on one fide alters from purplifh 



violet 



