MICROSCOPIC ILLUSTRATIONS. 285 



just as those arising from an object-glass will travel when 

 it is turned round. By this rule the imperfections of the 

 one may be distinguished from those of the other. In 

 the vision of luminous points an eye-glass of a crystallized 

 structure will often give an appearance of several points of 

 light which do not exist ; these, however, will always move 

 as the eye-glass is turned about. Defects in the polish of 

 eye-glasses may be tried by looking at them by oblique 

 candle-light. 



An inverting eye-piece cannot be made of more than two 

 glasses, without causing a dimness or muddiness in the 

 vision of opaque objects, and other deceptions, still more 

 disagreeable, with diaphanous bodies : the first of these is 

 a luminous spot in the centre of the field of view, which 

 seems to be an image of the object-glass produced by re- 

 flection from the numerous surfaces. This is most per- 

 ceptible when an object is placed in the centre of the field 

 of view, which only partially fills it, and happens at the 

 same time not to be very transparent in the middle. 

 When a candle is placed behind the stage, an image of it 

 may generally be seen in the axis of an engiscope, having 

 an eye-piece consisting of more than two glasses, and not 

 unfrequently a miniature image of the object viewed, if we 

 look a little obliquely into the instrument. If an engis- 

 cope, with a triple or quadruple eye-glass, is made to form 

 the image of a solar microscope, the aforesaid phenomena 

 are rendered very striking, from the intensity of the illu- 

 mination ; but they are not very perceptible when day- 

 light only is employed, and we merely look through the 

 instrument. 



Complicated eye-glasses, for the purpose of enlarging 

 the field of view, will, I think, almost invariably be found 



