8 On the UNEQUAL 
to be remarked.- Various attempts of this kind have been made 
by later philofophers and artifts. Indeed, the ftructure of the 
eye, compofed of folids and fluids varioufly combined, feems 
to prefent fo obvious and inftru@ive a pattern for imitation, 
that it is no wonder if the expectations entertained of the pro- 
duétions of art, rofe in proportion as they could be made to 
approach the conftruction of this exquifite model of Divine 
workmanfhip. 
Mr Dotionp’s firft experiments went no sabi than to 
prove to him, that glafs difperfes the heterogeneal rays of 
light more than water, when the refraétion-of the mean re- 
frangible ray is equal in both mediums. With thefe fcanty 
data, this able artift zealoufly went to work to conftruct tele- 
{copes on this new difcovered principle. But on this occafion 
his attempts were not attended with any degree of fuccefs. 
This need not much be wondered at. Befides the difficulty he 
‘mentions, arifing from the fpherical aberration, (which, by the 
by, if he had confidered the matter more attentively, he would 
have feen to be eafily furmountable) he would find between 
plate-glafs and water, but an inconfiderable difference of dif- 
perfive, power ; and if he made ufe of flint-glafs he would have 
all thofe difficulties to ftruggle with, which his fucceflors have 
not been able to remove, though fully apprized of their caufe. 
Turs want of fuccefs in his firft trials with fluids, and the 
difcovery he foon after made of a difference in the difperfive 
power of different kinds of glafs, which he was more fuccefs- 
ful in applying to the improvement of telefcopes, feems to have 
put an end to all thoughts of the ufe of fluids, nor has any 
thing of that kind been fince attempted, as far as I have been 
able to learn, fome unfuccefsful trials excepted, to conftruct 
thofe fmall perfpetives called opera glafles, on a plan fimilar to 
that of Mr Dotionp, by including fpirit of wine between 
two concave menifcufes of flint-glafs, the fluid fupplying the 
place 
