20 THE MICROSCOPE. 



of minute objects, which is the most important con- 

 tribution to the theory of the microscope that has yet 

 appeared, furnishing, as it does, an explanation of many 

 points which have hitherto greatly puzzled and per- 

 plexed microscopists, and showing that the conditions 

 of ordinary vision do not apply to objects of minute 

 size, so that microscopical vision is in this case a thing 

 sui generis^ in regard to which nothing can be legiti- 

 mately inferred from the optical phenomena connected 

 with objects of larger size. 



The essential point in the Abbe theory is that the 

 images of minute objects in the microscope are not 

 formed, as was formerly supposed, exclusively on the 

 ordinary dioptric method (that is in the same way in 

 which they are formed in the camera or telescope), but 

 that they are very largely affected by the peculiar 

 manner in which the minute constitution of the object 

 breaks up the incident rays, giving rise to diffraction. 



The phenomena of diffraction in general may be 

 observed experimentally by plates of glass ruled with 

 fine lines. Fig. 4 shows the appearance presented by 

 a single candle-flame seen through such a plate, an 

 uncoloured image of the flame occupying the centre, 

 flanked on either side by a row of coloured spectra of 



the flame, which become 

 dimmer as they recede 

 from the centre. A simi- 

 lar phenomenon may be 

 produced by dust scat- 

 tered over a glass plate, 

 and by other objects whose structure contains very 

 minute particles, the rays suffering a characteristic 

 change in passing 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 originating from a beam incident upon, for in- 

 stance, a diatom, appears as a fan of isolated rays, 



