366 B. T. LOWNE ON INTERFERENCE PHENOMENA. 



image of the bright opening, near the object-glass, and on either 

 side of this another image of the diaphragm corresponding to each 

 bright stripe on the object-glass ; that is, where the diffracted beams 

 do not interfere with each other. These images are necessarily 

 positive or bright, and lie nearly in the plane i h. As the light is 

 not homogeneous, and the violet rays are less diffracted than the 

 red rays, there is a spectrum instead of an image, corresponding to 

 the diffracted bright rays, with the red outside and the violet inside. 

 These are called diffraction spectra. The spectrum consists of 

 coloured images of the source of light. When the lines on the 

 plate are very close together the spectra are correspondingly 

 lengthened, and as the dark spaces equal the bright ones, the 

 narrower the spaces in the object the further apart the spectra are, 

 and the greater the aperture required to receive them. If the 

 grating consists of crossed lines the spectra form crosses ; if of 

 hexagonal fields or minute circular openings corresponding to the 

 centres of such hexagons there are six spectra in a circle, and so on. 



Now, it will be seen that the rays of light which produce these 

 images can have nothing whatever to do with the picture or 

 image (d) produced when the eye-piece is in its place. The light 

 proceeding from the diffraction spectra, towards the eye-piece, is 

 divergent light proceeding from the images of the diaphragm 

 in the plane i h. The light which gives us the image, which 

 we see with the eye-piece, is light converging from the object-glass 

 to the plane of the eye-glass. The field glass brings this convergent 

 light to a focus (d) between the field-glass and the eye-glass, in 

 the plane of the focus of the eye-glass. Try with an eye-piece to 

 see an object eight or ten inches in front of the field-glass, you 

 cannot. You can see nothing through an eye-piece used as a 

 telescope or magnifying glass, because it is adapted only for con- 

 verging rays, and no natural object gives out converging rays. 

 The focus of the eye-piece of a microscope is virtual ; that is, it is 

 nearer to the eye than the diaphragm between the field-glass and 

 the eye-glass. It is, as I have already stated, in the plane of the 

 eye-glass. 



Now, it is generally believed, since the publication of Abbe's 

 papers, that the appearance of the image seen depends on the 

 diffraction spectra. There could not be a more profound error. 



Professor Abbe says that if you block out these spectra you lose 

 the details in the image. This is true where the detail is due to 

 interference, as the image-forming light passes through the same 



