[ 28 9 ] 
n. 
A Letter from Mr. John Dollond to James 
Short, A. M. F. R. S. concerning a Mijlake 
in M. Euler ’ s Theorem for correSling the 
Aberrations in the ObjeSl-Glaffes of refract- 
ing Telefcopes. 
S I R, 
Read Nov. 23, f « \HE famous experiments of the 
17 52 A prifm, firft tried by Sir lfaac New- 
ton, fufficientiy convinced that great man, that the 
perfection of telefcopes was impeded by the different 
refrangibility of the rays of light, and not by the fphe- 
rical figure of the glades, as the common notion had 
been till that time 3 which put the philofopher upon 
grinding concave metals, in order to come at that by 
reflexion, which he defpair’d of obtaining by refrac- 
tion. For, that he was fatisfied of the impoflibility 
of correcting that aberration by a multiplicity of re- 
fractions, appears by his own words, in his treatife 
of Light and Colours, Book l. Pari 2. Prop. 3. u I 
“ found moreover, that when light goes out of air 
“ through feveral contiguous mediums, as through 
<c water and glafs, as often as by contrary refractions 
“ it is fo corrected, that it emergeth in lines parallel 
u to thofe in which it was incident, continues ever 
“ after to be white. But if the emergent rays be in- 
<c dined to the incident, the whitenefs of the emerging 
“ light will by degrees, in paffing on from the place 
“ of emergence, become tinged in its edges with co- 
“ lours. ’ 
It is therefore, Sir, fomewhat drange, that any 
body now-a- days fhould attempt to do that, which 
O o fo 
