S>8 



Microscopical E s s a ys, 



C H A P. IT.. 



Of Visign, of the Optical Effect of Microscopes^ 



AND OF THE MANNER OF ESTIMATING THEIR MAG- 

 NIFYING Powers. 



HP H E progrefs that has been made in die feienee of optics,, 

 in the laft and prefent century, particularly by Sir Ifaac 

 Newton, may with propriety be ranked among the greater! acqui- 

 fitions of human knowledge. And Meff. Delaval and Herfchel. 

 have fhewn by their difcoveries, that the boundaries of this, 

 feienee may be considerably enlarged. 



The rays of light, which minifter to the fenfe of fight, are the- 

 mod wonderful and aftonifhing part of the inanimate creation ; of 

 which we (hall foon be convinced, if we confider their extreme 

 minutenefs, their inconceivable velocity, the regular variety of 

 colours they exhibit, the invariable laws according to which they 

 are acted upon by other fubftances, in their reflections, inflections,, 

 and refractions, without the lea'ft change of their original proper- 

 ties ; and the facility with which they pervade bodies of the 

 greater!: denfity and clofeft texture, without refinance, without 

 crouding or difturbing each other. Thefe,I believe, will be 

 deemed fufficient proofs of the wonderful nature of thefe rays ; 

 without adding, that it is by a peculiar modification of them, that 

 we are indebted for the advantages we obtain by the mierofcope.. 



The- 



