t>34 THEORY OF MICROSCOPIC OBSERVATION. 



diffraction are stopped oft' in such a manner that the direct image 

 alone is perceptible on looking down the tube, the microscopic 

 image of the object will appear to be entirely blank, all fine 

 markings have disappeared precisely as though the instrument had 

 suddenly lost its whole optical power. If one of the diffraction 

 images be now brought into action as a second source of light, 

 the markings will immediately appear at right angles to 'the 

 line of union of the two effective sources of light, whilst they 

 remain as invisible as before in directions parallel to the line of 

 union. If we view Pleurosiyma attcnuatum, for instance, with an 

 objective of .about 3 mm. focal length and about 90 aperture, 

 there will be seen in the upper focal plane, outside the direct 

 aperture-image, four others due to diffraction, of which each 

 two diametrically opposed images correspond to a single system 

 of striae. According as the one or the other pair is stopped off, 

 the transverse or longitudinal stria 1 , will disappear in the image ; 

 and if all four are stopped off, absolutely nothing is seen of the 

 stria?. 



For the sake of simplicity we have throughout regarded the light 

 as homogeneous, and have taken the wave-length at *5 mic. As a 

 rule, the differently-coloured rays act together, and since the dis- 

 tances of the lines in the diffraction image above the objective are 

 proportional to the wave-lengths, the differently-coloured images 

 appear displaced laterally, and form a true spectrum in which with 

 favourable illumination separate Fraunhofer's lines are apparent. 

 Since all the diffraction pencils, which proceed from the same 

 surface-element of the object, nevertheless so far as the spherical 

 and chromatic aberrations can be disregarded intersect in the same 

 point, they produce at that point a colourless, and at the same time 

 sufficiently sharp, interference image. The limit of discrimination 

 may therefore in general be determined, because all the colours 

 take part in the production of the image that agree with the rays 

 of mean refrangibility, whose wave-lengths are actually about 

 *." mic. It may, however, occur in certain cases that rays of greater 

 refrangibility (e.g. green or blue) are the determining ones ; and in 

 photo-micrography, as is well known, wave-lengths come into 

 action which produce no visible effect on the naked eye. It would 

 therefore not be without interest to tabulate the wave-lengths of 

 thu different rays, and the limits of discrimination deduced from 

 them. As above shown, the values of the limits in (juestion 



