l8 ON INTERFERENCE IN THE MICROSCOPE. 



causes bright images, (of the diaphragm opening) images which 

 with homogeneous light, agree in size and form with the 

 direct geometrically-formed images. Any object moved backwards 

 and forwards between the mirror and the diaphragm opening, will 

 also appear amongst the diffraction images. In white light, 

 certainly, the effect is greatly modified in so far as the diffraction 

 images now appear coloured, because, as is well known, the position 

 of the maxima of brightness depends on the wave length, so that 

 the diffraction image of a circular opening in the diaphragm 

 produced in the focal plane of the objective appears radially 

 drawn out with its outer edges coloured red, its inner edges 

 blue. This complication strikes the eye at once, but in other 

 respects there is no difference between the conditions of an image 

 formed by white or by homogeneous light. We shall further 

 consider that the incident rays are strictly parallel, although the 

 before-mentioned diffraction images are clearly recognisable with 

 ordinary illumination (not strictly parallel rays.) 



For demonstration of the diffraction phenomena simple lined 

 objects are the most suitable, e.g., scales of Lepisma saccha/rinum. 

 Focus the instrument upon a lepisma scale, illuminated by central 

 light through a small diaphragm opening ; then remove the ocular 

 and examine the light effects in the upper focal plane of the objective. 

 The direct image of the diaphragm opening will be seen, and on 

 each side of it, in a line at right angles with the striae of the object, 

 the coloured diffraction images in symmetrical arrangement. If 

 the angle of aperture of the objective amounts to 60° the coarser 

 scales will shew several pairs of diffraction images 3 the finer 

 scales are, on the contrary, so finely striated that only the two 

 lying next to the direct image fall in the aperture of the objective. 



We have now to determine the effect of these diffraction 

 phenomena in the focal plane of the objective. The simplest 

 treatrr.ent of this subject will be to consider the aperture images 

 (both direct and diffraction) formed in the upper focal plane of the 

 objective as so many secondary sources of light, the interference 

 of whose rays takes place in the same manner as that which occurs 



