GENERAL DIRECTIONS. 



be the position of the sun in the heavens, such a position can 

 always be given to the plane of the mirror, that the light may be 

 reflected along the axis of the microscope. 



The great condensing lens is set in the larger end of the conical 

 tube T : and the lesser in the end of the cylindrical tube T' ; the 

 latter tube being moved within the former by an adjusting screw, 

 which appears at its side. By the second condensing lens, the light 

 is collected upon the opening in the slide, which is held between 

 two plates N, pressed together by spiral springs. 



The tube T' consists of two parts, one moving within the other, 

 like those of the telescope. 



The amplifying lenses are mounted in a brass ring, K, carried by 

 the upright piece, I, so that its optical axis shall coincide with 

 that of the illuminating apparatus. This optical part can be 

 moved to and from the object, by means of a rack and pinion, F, 

 attached to the piece H, which slides in the box G. 



The structure and principle of the instrument being understood, 

 it only remains to explain the method of using it. 



11. The room in which the operations are conducted should 

 have sufficient depth to allow the space between the microscope 

 and the screen, which t is necessary for the formation of an image 

 of the required magnitude. This space will vary with the mag- 

 nifying power required, but in general 10 or 12 feet beyond the 

 nozzle of the instrument is sufficient. The room should be rendered 

 as dark as possible, to give effect to the image, which, however well 



illuminated, is always incomparably less bright than would be 

 objects receiving the light of day. The window-shutters should 

 therefore be carefully closed, and all the interstices between them 

 stopped. If the room be provided with window-curtains, they 

 should be let down and carefully drawn. In a word, every means 

 should be adopted to exclude all light, except that which may 

 enter through the microscope. 



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