Microscopical Essays. i^j 



If an object, is fo opake as not to fuffer any light to pafs through 

 it, as much as poffible mud be thrown on it's upper furface by 

 that part of the apparatus which is peculiarly adapted for opake 

 objects. As the apertures of deep magnifiers are butfmall, and 

 confequently admit but little light, they are not proper for the 

 examination of opake objecls : this, however, naturally leads us 

 to our fecond head. 



Of the Management of the Light. 



The pleafure arifing from a juft view of a microfcopic object, 

 the diftin&nefs of vifion, &c. depend on a due management of 

 the light, and adapting the quantity of it to the nature of the 

 object, and the focus of the magnifier : therefore an object 

 mould always be viewed in various degrees of light. For, as 

 Dr. Hooke has well obferved, it is difficult to difimguilh in fome 

 objecls between a prominency and a depreffion, between a fhadow' 

 and a black ftain ; and in colour, between a reflection and a 

 whitenefs : a truth which the reader will find fully exemplified in 

 the examination of the eye of the libella, and other flies, which 

 will be found to appear exceedingly different in one pofition of 

 the light from what they do in another. 



The brightnefs of an object depends on the quantity of light ; 

 the diftinctnefs of vifion, on regulating the quantity to the object. ; 

 for fome will be loft, and drowned, as it were, in a quantity of 

 light that is fcarce fufficient to render another viable. A ftrong 

 light may be thrown on an objeft various ways ; firft, by means 

 of the fun and a convex lens ; for this purpofe, place the micro- 

 fcope about three feet from a fouthern window ; take a deep 



I . convex 



