66 



VISION WITH THE COMPOUND MICROSCOPE 



central image will be clear and uncoloured, but it will be flanked 011 

 either side by a row of coloured spectra of the flame which are fainter 

 and more dim as they recede from the centre : fig. 48 illustrates this. 

 A similar phenomenon may also be produced by dust scattered 

 over a glass plate and by other objects whose structure contains very 

 minute particles, the light suffering a characteristic change in pass 



ing through such objects, that change 

 consisting in the breaking up of a 

 parallel beam of light into a group of 

 rays diverging with wide angle, and 

 forming a regular series of maxima and 

 minima of intensity of light due to difference of phase of vibration. 



In the same way in the microscope the diffraction pencil origi- 

 nating from a beam incident upon, for instance, a diatom appears 

 as a fan of isolated rays, decreasing in intensity as they are further 

 removed from the direction of the incident beam transmit ted through 

 the structure, the interference of the primary waves giving a number 

 of successive maxima of light with dark interspaces. 



With daylight illumination if a diaphragm opening be interposed 

 between the mirror and a plate of ruled lines placed upon the stage, 

 the appearance shown in fig. 49 will be observed at the back of the 



objective on removing the eye-piece and 

 looking down the tube of the microscope. 

 The central circle is an image of the dia- 

 phragm opening produced by the direct, 

 so-called non-diffracted rays, while those 

 on either side are the diffraction images 

 produced by the rays which are bent off 

 from the incident pencil. In homogene- 

 ous light the central and lateral images 

 agree in size and form, but in white light 

 the diffracted images are radially drawn 

 out with the outer edges red and the 

 inner blue (the reverse of the ordinary 

 spectrum), forming, in fact, regular spec- 

 tra, the distance separating each of which varies inversely as the 

 closeness of the lines, being, for instance, with the same objective 

 twice as far apart when the lines are twice as close. 



The formation of the microscopical image is explained by the 

 fact that the rays collected at the back of the objective, depicting 

 there the direct and spectral images of the source of light, reach in 

 their further course the plane which is conjugate to the object, and 

 give rise there to an interference phenomenon (owing to the connec- 

 tions of the undulations), this interference effect giving the ultimate 

 image which is observed by the eye-piece, and which therefore 

 depends essentially on the number and distribution of the diffracted 

 beams which enter the objective. 



It would exceed the limits and the object of this handbook to 

 attempt a theoretical demonstration of the action of diffraction 



FIG. 49. 



so as 



spectra in forming the images of fine structure and striation 



to afford ' resolution.' Those who desire to pursue this part of the 



