ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 235 



the vertical pinion ; H, small mirror, universally adjustable ; I, 35 mm. 

 lens (Zeiss planar). 



The author gives several examples of his results, one of which is 

 shown in fig. 32 (pi. IV.). 



(5) Microscopical Optics and Manipulation. 



Standard Measurement in Wave-lengths of Light.* — The 

 principles underlying A. E. Tutton's method of interference measure- 

 ments were described in the February number of this Journal, and a 

 later description of the apparatus was promised.f 



A general view of the interferometer and one of the duplicate 

 Microscopes of the comparator, together with sufficient of the bar- 

 carriage to enable some idea of the whole apparatus to be gained, is 

 now given in the accompanying illustration (fig. 33), together with 

 the author's description. 



The whole instrument is mounted on a large stone block, resting on 

 isolated concrete foundations. On a small stone pedestal, similarly 

 isolated, in front of the large block, rests the pedestal of the auto- 

 collimating telescope and attached Geissler tube of the interferometer. 

 In the common focal plane of the telescope objective and eye-piece, 

 opposite the junction of this main optical tube with the rectangularly 

 attached side-tube carrying the Geissler tube, a small totally reflecting 

 prism is arranged, half covering the focal aperture. A still smaller 

 rectangular stop or opening in a plate in front of, and almost touching, 

 that one of the perpendicular prism faces which is directed towards the 

 objective, and lies in the focal plane very close to the edge, dividing the 

 closed half from the open half, is the effective source of the interfering 

 light ; the rays from the Geissler tube, received on the other face of the 

 right-angled prism, are arranged to fill this stop after reflection from 

 the hypotenuse of the prism. The rays proceed from the stop to the 

 objective, which they are arranged to fill with light, and thence pass out 

 of the telescope as parallel rays, in the path of which the dispersion and 

 interference apparatus is placed. The rays return to the telescope from 

 the latter along practically the same path, but after re-entering the tele- 

 scope, instead of returning to the little rectangular stop, their origin, 

 they are deflected just sufficiently to one side to form an image of the 

 stop, the same size as the original, in the open semicircular aperture of 

 the focal plane, within a couple of millimetres of the real stop. This 

 closeness to identity of path of the outgoing and incoming rays, and 

 consequently normal incidence on the reflecting glass surfaces of the 

 interference apparatus, is largely responsible for the magnificent field of 

 parallel straight-lined interference bands which the author's interfero- 

 meter affords, for it fulfils an essential condition for perfect interfer- 

 ence. 



With the ordinary eye-piece in position, the images of the stop 

 reflected from the various surfaces of the interference apparatus can be 

 focused, adequately magnified, and viewed during their adjustment to 



* See this Journal, 1910, pp. 107-8. 



t Tom. cit., p. 107 ; Phil. Trans. A, ccx. (1910) p. 1 ; Nature, lxxxii. (1910) pp. 

 338-41 (1 fig.). 



It 2 



