Eyes and other Objects. 73 



but one window in the room, or to darken the others ; 

 to choose one which commands a few conspicuous 

 objects, and to close its shutters slightly, so as to 

 avoid the glare from their panels. The microscope 

 must be raised on a box, and put as near to the 

 horizontal position as it can be without causing the 

 slide to fall against the object-glass ; or this result may 

 be prevented by fastening the slide to the stage with 

 elastic bands, or an atom of wax. I have tried to 

 devise a plan for showing the action of the lenses by 

 lamp-light ; and can recommend that of inclining the 

 microscope raised on a box as for the daylight experi- 

 ment, and looking directly at the shade of a modera- 

 teur lamp, on which some device, cut out in black 

 paper, is temporarily attached for instance, two 

 initial letters, about an inch and a half in height, cut 

 from a printed placard. They are to be seen repeated 

 in beautiful clearness all over the object, and like the 

 little views, in their true position, not inverted, for the 

 very interesting reason that each lens is acting as an 

 additional object-glass to the microscope. 



A similar experiment may be shown with the eyes 

 of most other insects, but scarcely so conveniently 

 with any as that of the dragon-fly, its lenses, especially 

 at the front of its eyes, being larger than those of 

 most others. A considerable difference will be found 

 in the number of lenses belonging to insects' eyes. 

 It is calculated that the ant has 50 lenses, the house- 

 fly 4000, while as many as 34,650 have been counted 

 by the naturalist Geoffroy in the eyes of a but- 

 terfly. 



