Microscopical Essay 14 



From this period, to the year 1736, the microfcope appears 

 not to have received any conhderable alteration, but the 

 fcience itfelf to have been at a (land. The improvements which 

 were making in the reflecting telefcope, naturally led thofe who 

 had turned their thoughts this way, to ex peel a fimilar fervice 

 for microfcopes on the fame principles : accordingly we find two 

 plans of this kind ; the firft was that of Dr. Robert Barker. 

 This inftrument is entirely the fame as the reflecting telefcope,. 

 excepting the diftance of the two fpeculums, which is lengthened, 

 in order to adapt it to thofe pencils of rays which enter the 

 telefcope diverging; whereas, from very diftant objects, they 

 come in a direction nearly parallel. But this was foon laid afide, 

 not only as it was more difficult to manage, but alfo becaufe it 

 was unfit for any but very fmall or tranfparent objects : for the 

 object being between the fpeculum and the image, would, if it 

 were large and opake, prevent a due reflection of light on the 

 object. 



The fecond was contrived by Dr. Smith.* In this there were 

 two reflecting mirrors, one concave, and the other convex ; the 

 image was viewed by a lens. This microfcope, though far from 

 being executed in the belt manner, performed, fays Dr. Smith,, 

 very well, fo that he did not doubt but what it would have ex- 

 ceiled others, if it had been properly finifhed. 



As fome years are more favorable to the fruits of the earth, fo 

 alfo fome periods are more favorable to particular fciences, being 

 rich in difcovery, and cultivated with ardor. Thus, in the year 

 6 *73 8 » 



* Dr. Smith's Optics, Remarks, p. 94. 



