ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 349 



greater than the length of the sound-waves. If the obstacle is reduced 

 in size the waves pass completely round it, and there is no shadow, 

 or if the notes are of higher pitch so that the waves are reduced in 

 size a smaller obstacle than before will produce the shadow. In the 

 case of liglit, when the objects are large in comparison with the 

 wave-lengths, shadow elfects, of course, result, that is, we have a 

 rectilinear propagation of the light ; but when they are reduced to 

 smaller and smaller dimensions they are not now many multiples but 

 only a few multiples of tlie wave-lengths, and we have no longer 

 rectilinear propagation,- -d/o2J<r^caZ considerations are not therefore 

 applicable, and we have to refer to the laws of diffraction, i. e. those 

 laws which explain the changes produced in the rays of light in 

 consequence of their interception or unequal retardation by minute 

 particles. In now explaining microscopical vision, it is necessary, 

 therefore, to take into account that the images of 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 the microscopical 

 image is very largely affected by the peculiar manner in which the 

 minute constitution of the object gives rise to the phenomena of 

 diffraction. 



It is hardly necessary to say that Professor Abbe's discovery con- 

 stituted a most important innovation upon the views formerly current 

 (as much so as the spectroscope and the telephone in chemistry and 

 physics). The honour of the discovery belongs to him alone, no 

 hint of it having been previously given, and no one having even seen 

 what every one can now easily see, a " diffraction spectrum " in the 

 Microscope. 



(a) Minute Objects. 



The phenomenon of diffraction in general may be observed experi- 

 mentally, as is well known, by plates of glass ruled with fine lines. 

 Fig. 91 shows the appearance presented by a single candle-flame seen 



Fig. 91. 





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 grow dimmer as they recede from the centre. A similar pheno- 

 menon 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 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. The formula in which these phenomena are 

 expressed is that known as Fraunhofer's formula, Avhich was first 

 published in 1821.* 



* Deukschr. k. Bayer. Akad,, viii. 



Ser. 2.— Vol. 1. 2 B 



