o^O 



ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 613 



In ruby glass, for instance, the colour is due to minute particles of gold 

 diffused through the glass, so small as to be beyond the powers of the 

 Microscope as ordinarily used. By special methods of illumination, 

 however, at right angles with the optical axis of the Microscope, and by 

 limiting the plane of such illumination, the particles come into view as 

 diffraction discs. Mr. Gordon then dealt with some experiments of his 

 own, originally suggested by a paper of Lord Rayleigh's, but which 

 were still incomplete, which consisted especially of a method of lighting 

 up the object by means of diffracted light, the principle being explained 

 by a diffraction slit formed by the edges of two knives stuck in a board 

 so that their edges overlapped towards the points, but were about an 

 eighth of an inch apart near the handles. It was such a piece of 

 apparatus that Sir Isaac Newton worked when he made his first precise 

 recorded observations on the subject of diffracted light. Mr. Gordon 

 referred to the observation of Helmholtz, as far back as 1874, that the 

 limit of a useful power in a high-power objective is reached when the 

 lens of the objective is of such focal length that its diameter is rather less 

 than the diameter of the pupil of the eye, and that beyond that point- 

 there was no advantage in increasing the magnifying power of the 

 objective, but that further magnification was best obtained by increasing 

 the power of the eye-piece. But this method had also drawbacks owing 

 to the smallness of the emergent pencil of light ; such, for instance, as 

 the greater prominence of dust upon the lens or of floating particles in 

 the eye. Mr. Gordon considered that this was responsible for the 

 limitation of magnifying powers at present in use by microscopists to 

 1500 or 2000 diameters, whilst most good work was done with magnifi- 

 cations of from 400 to 600 — a statement, however, which surely needs 

 some qualification, whatever may be the incidental disadvantages due to 

 high eye-piecing. However, Mr. Gordon's method of getting over the 

 difficulty is by the interposition in the tube of the Microscope of a ground- 

 glass screen on which the image is received from the objective, so as to 

 scatter the incident rays of light, the screen being made to oscillate in 

 order to prevent its grain from becoming visible and so impairing the 

 details of the picture. This picture can then be magnified again by 

 means of a second Microscope in place of an ordinary eye-piece, with 

 consequent greatly increased magnification. It may not perhaps be 

 superfluous to recall that the mere magnification of an object, or even the 

 rendering visible of what could not otherwise be seen to be existent, as 

 under Siedentopf's experiment, does not give any optical solution as to 

 its true shape and size. In fact, it has been mathematically proved, and 

 remains true, to quote Lord Rayleigh's own words, " In the Microscope 

 there is nothing except lack of light to hinder the visibility of an object 

 however small. But if its dimensions be much less than half a wave- 

 length, it can only be seen as a whole, and its parts cannot be distinctly 

 separated, although in cases near the border-line some inference may 

 possibly be founded upon experience of what appearances are presented 

 in various cases. . . . What has been said about a luminous point 

 applies equally to a luminous line. If bright enough it will be visible, 

 however narrow ; but if the real width be much less than the half wave- 

 length, the apparent width will be illusory." 



