﻿Observations on Diffraction. 75 



piece. In practice, however, the first spectra are necessary, 

 and perhaps often sufficient, to show the detail : sometimes 

 the higher spectra may be needed to add breadth to the 

 shadows. In fact, from ordinary objects rays of all degrees of 

 obliquity are available to mark the shadows, but from objects 

 capable of diffraction there is no degree of obliquity less than 

 that which tends towards the first spectrum, and the aperture 

 of the objective must not be too narrow to receive these rays. 



The question naturally arises, how far it is possible to know 

 whether an image is a reproduction of the general form of 

 the original object or a diffraction modification : so far as my- 

 experience goes, the two appearances may be distinguished. 

 Of course in perfect theory, even if an image could be saved 

 from spherical and chromatic aberration, it must suffer 

 something from the waves : the image of a star in a telescope 

 must be enlarged on this account ; but there is a great differ- 

 ence between this falsification and that which arises when the 

 eyepiece is taken out of focus. Fig. 82 is a species of the 

 diatom Triceratium favus : it appears as a spotted framework 

 of ill-defined outlines with little groups in the spaces: but the 

 microscope can be adjusted so as to show a well-defined honey- 

 comb with clear hexagonal spaces. In an experiment de- 

 scribed above, figures known to be circles are reproduced in 

 their own true form when at one position with regard to a 

 convex lens, and more elaborate ili-focussed forms at positions 

 on either side of the former : it is reasonable, then, to apply 

 this test to distinguish between several images of an unknown 

 original. No doubt the simpler form, which is taken to be 

 the true one, might often be resolved into something more 

 complex by a stronger objective, but this one in its new 

 conditions would again be simple and well-defined amidst its 

 possible diffraction derivatives. 



I have not sufficient familiarity with the use of the highest 

 microscope powers, and can imagine that the foregoing tests 

 are not easy, if indeed possible, to apply in such cases ; but 

 I suppose that with ^-inch and lower powers there is no need 

 for uncertainty as to the practical form of the original object. 



FresneFs Diffraction from Geometrical Figures. 



Some interesting details can be observed from the shadows 

 about other figures of Bridge's series. It may be here noted 

 that such objects may be made by drawing rather thick 

 figures in ink upon a card about 1 foot square and reducing 

 by photography to 1 inch square : the negatives so produced 

 give the desired transparent lines on a dark ground : it will 

 be understood that a circle means a circular line of lioht in 



