4 THE AMERICAN MONTHLY [J an 



FXY is relatively narrow to the cone FAB. As a matter 

 of fact, condensers are always made of large "aperture," 

 they are always made to give a large cone of light, and 

 it is usual to narrow them down when desired by an iris 

 diaphragm. But objectives are made of fixed "apertures" 

 which ditfer very greatly, according to the power of the 

 objective, a low power like a 1" objective having an "ap- 

 erture" much smaller than a £" objective for instance — or 

 to put it in other words, the "cone of light" admitted by 

 a 1" objective is relatively small as compared to that ad- 

 mitted by a £". 



Now as regards the object, there was light knocking up 

 against it from all sides — partly blue light, partly red — 

 and as we may for our purpose regard an object as a col- 

 lection of little prisms of various shapes, which turn and 

 twist the light falling on them in various directions ac- 

 cording to the law of refraction and reflection, we can 

 easily see that a lot of this light which hit the object, had 

 its direction changed so as to fall within the cone of light 

 admitted by the objective. So that plenty of the red 

 lierht. which would otherwise have passed outside of the 

 objective, has now been thrown up into it by the object, 

 and depicts the object in that color in consequence. *Fig. 

 3 illustrates this. Of course this has not prevented the 

 blue light also forming an image of the object in blue, but 

 you will notice that the difference in area between the red 

 and blue portion of the disc (fig. 1 a) is considerable, in 

 fact the former is about eight times as great as the latter, 

 and as a consequence the red image will be roughly speak- 

 ing about eight times as strong as the blue one, which 

 causes the latter to be swamped out as far as the eye is 

 concerned. 



We will now proceed to a different method of illumin- 

 ation. The above method was limited to the use of ob- 

 jectives of not higher power than £", but the illumination 

 now to be described, as well as the third method, is ap- 



