1900] MICROSCOPICAL JOURNAL. 3 



however, in making the discs that the central spot which 

 gives the color to the background is comparatively darker 

 than the other part of the disc. 



Now we will vary the experiment and make a disc like 

 fig. 1 d, of four sectors ; two opposite ones red, the other 

 two blue, and on it paste a black central spot. Using the 

 disc we will look at a small piece of silk mounted in Can- 

 ada balsam (a morsel of a Japanese silk handkerchief does 

 excellently) and notice the result : It is as if the silk 

 had been woven in two colors — all the horizontal threads 

 composing the warp being blue ; all the vertical ones 

 which form the weft being red. 



Having made these experiments let us stop and consider 

 the cause of those rather startling results. Why did the 

 object ostentatiously appropriate to itself all the red light, 

 and why did the background appear blue ouly when we 

 used the disc like fig. la ? The answer to the last ques- 

 tion is simply that without having an object in the field 

 no red light gets into the microscope tube, because the 

 cone of light admitted by the 1" objective is no larger 

 than the cone of light coming from the condenser through 

 the blue central portion of the color disc, as seen iu fig. 2. 



THe expressions, viz : — "Aperture" of an objective or 

 condenser, and "Cone of Light" admitted by an objective 

 or condenser are of somewhat frequent recurrence in this 

 article. It may be useful to some readers to have these 

 terms explained, an exact comprehension being essential 

 to understand the subject. When we use a condenser we 

 bring the light which passes through it to a focus on the 

 object ; and the shape of the passage of light is a cone 

 with its apex at the object and its base the condenser 

 lenses. The cone is represented by the triangle FAB of 

 fig. 2. After passing the focus the light again forms a 

 cone, inverted this time, and the whole or some part of 

 this cone may be taken up by the objective (FXY fig. 2). 

 The cone may be wide or narrow; for instance the cone 



