14 



Microscopical Essays. 



the refraclivc power of water is lefs than that of glafs, the fe 

 globules do not magnify fo much as thofe of the fame fize, 

 which are made of glafs : this was alfo contrived by Mr. Gray. 

 The fame ingenious author invented another water microfcope. 

 confifiing of two drops of water, feparated in part by a thin brafs 

 plate, but touching near the center; which were thus rendered 

 equivalent to a double convex lens, of unequal convexities. 



Dr. Hooke defcribes a method of ufmg the fmgle microfcope, 

 which feems to have a great analogy to the foregoing methods 

 of Mr. Gray. If you are defirous (he fays) of obtaining a mi- 

 crofcope with one fmgle refraction, and confequently capable of 

 procuring the greateft clearnefs and brightnefs any one kind 

 of microfcope is fufceptible of ; fpread a little of the fluid you in- 

 tend to examine on a glafs plate, bring this under one of your 

 microfcopic globules, then move it gently upwards, till the fluid 

 touches the globule, to which it will foon adhere, and that fo 

 firmly, as to bear being moved a little backwards or forwards. 

 By looking through the globule, you will then have a perfect view 

 of the animalcule in the drop. * 



Having laid before the reader the principal improvements that 

 have been fuggefted or made in the fmgle microfcope, it remains 

 only to point out thofe inftruments of this kind, which, from the 

 mode in which they are fitted up, feem beft adapted for general 

 ufe ; the peculiar advantages of which, as well as the manner of 

 ufmg them, will be defcribed in the third chapter of this work. 



Fig. 



* Hooke's Lectures and Conjectures, p. 98. 



