2G2 



KOODj ON THE PilACTICAL APPLICATION OV 



by a blacked pasteboard tube. Much vexation will be avoided 

 by coustructiiig at the outset the arrangement seen in the 

 woodcut, Fig. 1. Blocks are fitted around the foot of the 



Tirr. 1. 



microscope, that it may be firmly held in position, and the 

 camera box slides between parallel strips of board, so that its 

 distance from the microscope can be varied. The length of 

 A a' is seven feet ; the frame holding the ground glass slides 

 in at G ; behind it, at l, is a door on hinges, carrying an 

 achromatic lens of two inches focal length, for the purpose of 

 magnifying the image on the ground glass while focusing. 

 The glass plate should be finely ground. A tube, lined with 

 black velvet, is to be inserted in the compound body, as re- 

 commended by Shadbolt, if the eye-piece is not employed. 

 Precautions must, of course, be taken that light does not enter 

 at unguarded points. At f is a rod connected with a flap of 

 blackened sheet brass in the interior of the box, with which 

 the exposure of the sensitive plate is very conveniently effected. 

 It is obvious that, while the operator is manipulating the 

 mirror, or using the stage movements, on account of the 

 length of the apparatus it is impossible for him to see the 

 ground glass, or even to know when light has been thrown on 

 it. A plane mirror, monnted as seen at m, reflects the image 

 of the ground glass, enabling him not only to arrange the 

 illumination with nicety, but to select the microscopic object, 

 and to focus on it approximately. While the mirror is in use 

 the door carrying the achromatic lens stands open ; the mirror 

 is afterwards removed, and the focal adjustment completed, 

 with the help of the lens, by the rod and lever attached to the 

 rack work of the microscope. If the rack work is moderately 

 good, this arrangement is very delicate. When a high mag- 

 nifying power is employed, it is essential that the microscope 

 be proxided 'odth stage movements, to bring the object into 

 its proper position. The lever stage is not to be recommended 

 for this purpose. 



Illumination of the object. — That direct sunlight is greatly 

 to be preferred is admitted by those who have experimented 



