8 THE AMERICAN MONTHLY [Jan 



there is nothing to compare with these for experimental 

 purposes. Then we remove the ohject, and carefully focus 

 our coudenser on to the same plane. If the condenser is 

 properly focussed, the back lens of the objective should 

 be filled with light when the eyepiece of the microscope is 

 taken out and we look down the tube. Next, we close the 

 iris diaphragm of the condenser, looking the while down 

 the tube, till all the light is cut off from the red part of 

 the disc, and only just tills the blue central part (fig. 7). 

 Now we can replace the eyepiece, bringing the object in 

 position again, and the object will appear clearly and dis- 

 tinctly red on a blue ground, and we shall notice that the 

 diatoms appear to stand out almost stereoscopically, and 

 that the thicker parts of the bone section, which appear 

 hazy if looked at with the same objective without a color 

 disc, have become much better defined. Of course with 

 a color disc having a blue central spot without the red 

 rim the object shows up in its natural color on a blue 

 ground. Vice vert>a, if the disc is completely blue with 

 only a central spot uncolored (the size of which, however, 

 must not exceed one-sixth to one-eighth part of the diam- 

 eter), the object shows up blue on an uncolored ground. 



The diffraction of light passing by and through the ob- 

 ject is the chief cause of our results in this instance. 

 Without going too deeply into an explanation of diffrac- 

 tion, which would involve discussing the laws of wave 

 motion, 1 need only mention that whenever a ray of light 

 (R. fig. 6) meets an obstacle (S, fig. 6), this point becomes 

 a centre for waves to spread out from. Now the crests 

 and hollows of all the waves of light produced at this 

 point and in the immediate vicinity intermingle and in- 

 teract on one another, and thereby become regulated in 

 such a way that one portion of the light, R', travels on 

 undisturbed as a continuation of the original ray, whilst 

 other diffracted rays (D 1, D 2, D 3) are produced (dif- 

 fering only from the last-named one in that they are not 



