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Microscopical Essays. 



convex lens, that is mounted in a femicircle and fixed on a Hand, 

 fo that it's pofition may be eafily varied ; place this lens between 

 the objecl; and the window, fo that it may collect a considerable 

 number of the folar rays, and refracl; them on the objecl;, or the 

 mirror of the microfcope. If the light thus collected from the 

 fun is too powerful, it may be tempered, by placing a piece of 

 oiled paper, or a glafs lightly greyed, between the objecl; and the 

 lens : by this means, a convenient degree of light may be obtain- 

 ed, and diffufed in an equal manner over the whole furface of an 

 objecl;, a circumftance that mould be particularly attended to ; 

 for if the light be thrown in an irregular manner, that is, larger 

 portions of it on fome parts than on others, it will not be 

 diftin&Iy exhibited. 



Where the folar light is preferred, it will be found very con- 

 venient to darken the room, and to reflect the rays of the fun on 

 the above-mentioned lens, by means of the mirror of a folar micro*- 

 fcope fitted io the window-mutter; for by this apparatus the 

 obferver will be enabled to preferve the light on his objecl;, nof^ 

 withstanding the motion of the fun* 



Cutting off the adventitious light as much as poffible, by 

 darkening the room where you are ufmg the microfcope, and 

 admitting the light only through a hole in the window-mutter, or 

 at mod keeping one window only open, will alfo be fpund very 

 conducive towards producing a diftincl; view of the objecl;. 



As the motion of the fun, and the variable flate of our atmo- 

 fphere, renders folar obfervations both tedious and inconvenient, 

 it will be proper for the obferver to be furnifhed with a large tia 



Xanthoma. 



