8 On the UNEQUAL 



td 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 inftrudUve a pattern for imitation, 

 that it is no wonder if the expectations entertained of the pro- 

 ductions of art, rofe in proportion as they could be made to 

 approach the conftruclion of this exquifite model of Divine 

 workmanfhip. 



Mr Dollond's firft experiments went no farther than to 

 prove to him, that glafs difperfes the heterogeneal rays of 

 light more than water, when the refraction 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- 

 fcopes 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 fuccefTors have 

 not been able to remove, though fully apprized of their caufe. 



This 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 perfpectives called opera glaffes, on a plan fimilar to 

 that of Mr Dollond, by including fpirit of wine between 

 two concave menifcufes of flint-glafs, the fluid fupplying the 



place 



