Microscopical Essays. 29 



I Thefcience of optics, which explains and treats of many of 

 the properties of thefe rays of light, is deduced from experiments, 

 on which all philofophers are agreed. It is impoffible to give an 

 adequate idea of the nature, of virion, without a knowledge of 

 thefe experiments, and the mathematical reafoning grounded 

 upon them; but as to do this would alone fill a large volume, I 

 mall only endeavour to render forne of the more general princi- 

 ples clear, that the reader, who is unacquainted with the fcience 

 of optics,' may neverthelefs be enabled to comprehend the nature 

 of vifion by the micro fcope. Some of the mod important of 

 thefe principles may be deduced from the following very 

 Miterefting experiment. 



Darken a room, and let the light be admitted therein only by 

 a fmallhole; then, if the weather is fine, you will fee on the 

 wall, which is facing the hole, a piaure of all thofe exterior ob- 

 jefts which are oppofite thereto, with all their colours, though 

 thefe will be but faintly feen. The image of the objeas that are 

 Stationary, as trees, houfes, &c. will appear fixed; while the 

 images of thofe that are in motion, will be feen to move. The 

 image of every objea will appear inverted, becaufe the rays crofs 

 each other in palling through the fmall hole. If the fun mines on. 

 the hole, we (hall fee a luminous ray proceed in a (trait line, and 

 terminate on the wall. If the eye is placed in this ray, it will be in 

 a right line with the hole and the fun: it is the fame with every 

 other objea which is painted on the wall. The images of the ob- 

 jeas exhibited on the fame plane, are fmaller in proportion as the 

 objeas are further from the hole. 



Many 



