OPTICS. 293 



The short tube E, which the microscope is 

 screwed to, enables you, by sliding it backwards or 

 forwards in the other tube D, to bring the objects 

 to their true focal distance ; which will be known 

 by the sharpness and clearness of their appearance; 

 they may also be turned round by the same means, 

 without being in the least disordered. 



The magnifiers most useful in the solar micro- 

 scrope are, in general, the fourth, fifth, or sixth. 



This microscope is the most entertaining of any. 

 There are also several conveniences attending it, 

 which no other microscope can have; for the 

 weakest eyes may use it without the least straining 

 or fatigue: numbers of people may view any object 

 together at the same time. Such, too, as have 

 a little skill in drawing, may, by this contrivance, 

 easily sketch out the exact form of any object 

 they have a mind to preserve a figure of; since 

 they need only fasten a paper upon the screen, 

 and trace it on it, either with a pen or pencil. 



For this purpose a frame might be made, in 

 which a sheet of paper may be put in or taken out 

 at pleasure ; for if the paper be single, the image 

 of an object will be seen as plainly almost on the 

 back as on the other side; and by standing behind 

 the screen, the shade of the hand will not obstruct 

 the light in drawing, as it must in some degree 

 when one stands before it. 



It must be observed, that formerly the solar 

 microscope had no looking-gJass belonging to it, 

 and, therefore, was of use a few hours only in a day, 

 when the tube could be placed directly against the 

 body of the sun, and even then not without a 

 good deal of trouble: but by this lucky con- 

 trivance of a looking-glass, the sun's rays may be 

 reflected through the tube, whatever its height or 



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