360 Transactions of the Society. 



of the real image, instead of being intercalated between them. It is 

 quite obvious that a bright line of the hypothetical diffraction pattern 

 must occupy the middle of the field, the point marked B in Naegeli 

 and Schwendener's diagram. Is it also plain that this spot must be 

 occupied by a bright line in the visible image of the grating ? The 

 simple answer to this question is that, if such a bright line did chance 

 to occupy that position, it could easily be displaced by a turn of the 

 stage-propelling screw of the Microscope. What, then, would become 

 of the coincidence between the "diffraction image" and the so-called 

 dioptric image ? Moving the diffraction grating across the stage of the 

 instrument would not displace the images produced by diffraction of the 

 source of light. Of course, if the grating be moved out of the beam, 

 the diffraction images will disappear, just as the shadows in a room 

 disappear when the blind is drawn. But they will be blotted out one 

 by one where they stand ; they will not shift, any more than shadows 

 of the window-frame upon the wall of a room will shift at the drawing 

 of the blind. Thus the extremely simple experiment of sliding the 

 Diffractions Platte across the stage of the Microscope suffices to dis- 

 tinguish what is due to refraction from whatever is due to interference. 

 The "dioptric image" will be displaced by the displacement of the 

 object on the stage ; the interference image — if there be one — will 

 remain stedfast. Make this experiment with your eye upon the 

 microscopic image of the grating, and you will see that everything, 

 down to and including its minutest detail, is part of the "dioptric 

 image," and obeys the law of refraction. Kemove the ocular and 

 repeat the experiment with your eye on the diffraction images in the 

 principal focal plane at the back of the objective, and you will see 

 that they remain unmoved by the displacement of the grating until 

 one by one they are extinguished as it passes out of the beam, and 

 that then they die at their posts. What is true of this primary set 

 of diffraction images must be true also of any secondary interference 

 images derived from them. They are all ex hypotliesi images of the 

 source of light, and while it retains its position they cannot be 

 displaced. 



This experiment may be reversed. Leaving the Diffractions 

 Platte at rest upon the stage, you move the source of light across the 

 field of the instrument. Its images are, of course, displaced in 

 the opposite direction. You then replace the ocular, and observe the 

 image of the grating while you again displace the source of light. 

 There is no corresponding movement. The distribution of light is 

 affected, and the centre of illumination follows the inclination of the 

 axis of the beam, but the bright lines of the image, the minute details 

 said to be produced not by refraction but by interference, remain. 

 They obey, with a perversity which, however, does not surprise you, 

 the laws of dioptrics, and exhibit no sympathy with the diffraction 

 system. 



It is indeed obvious that the position of the hypothetical bright 



