ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 107 



carriage for the mirror or flame, or other source of light whose spectrum 

 is to be examined. A rod h h serves to focus S. The author gives a full 

 account of the adjustment necessary. He also gives some examples of 

 the satisfactory results obtained. 



Wave-length Comparator for Standards of Length.* — A. E. H. 

 Tutton, in describing his instrument for comparing standards of length 

 — the Imperial Standard yard, for instance, with official or other copies — 

 states that it is the most perfect instrument yet devised for measure- 

 ment of wave-lengths in general. The principle of the instrument is 

 an improved form of the author's interferometer described to the Royal 

 Society in 1898. The essential point of the instrument is that one of 

 the two Microscopes, employed to focus the two defining lines on a 

 standard yard bar, actually carries just above the objective one of the 

 two glass plates of the interference apparatus, which reflect the mono- 

 chromatic light (hydrogen or cadmium red radiation) which is caused 

 to interfere and produce rectilinear dark bands. When the Microscope 

 is moved the plate consequently moves with it, and the amount of move- 

 ment is absolutely afforded by the movement of the interference bands, 

 being equal to half the wave-length of the light employed for every 

 band which passes the reference spot in the centre of the held of the 

 interferometer telescope. So perfectly has this fine movement been 

 achieved that the Microscope and the bands can be caused to move 

 simultaneously by rotation of the large fine-adjustment wheel, so steadily 

 that each band can be made to pass the reference spot as slowly as one 

 wishes and be arrested instantly, without the slightest tremor, at any 

 fraction of its width, so that the control of the bands and the counting 

 is a perfectly simple matter. 



In order to compare two standard bars it is only necessary (1) to 

 place the bar of known length, supported on an elaborate mechanism 

 for the adjustment of the bars, under the two Microscopes, carried on 

 massive yet delicately moving sliders on a 6-foot V-and-plane bed, so 

 that the two defining lines are adjusted between the spider-lines of the 

 micrometer eye-piece in each case ; (2) to replace the standard by the 

 copy to be tested, so that the defining line near one end is similarly 

 adjusted under the corresponding Microscope ; then, if the other de- 

 fining mark is not also automatically adjusted under the second Micro- 

 scope which carries the glass interference plate, as it should be if it is 

 an exact copy, (3) to traverse that Microscope until it is so adjusted ; 

 and (4) to observe and count the number of interference bands which 

 move past the reference spot during the process. The difference between 

 the bars is this number multiplied by the half-wave-length of the light 

 in which the bands are produced. The temperature of the whole room 

 is controlled entirely electrically, being maintained constant at the official 

 temperature, 62° F. (A description of the apparatus will appear later.) 



Use of Wave-length Rulings as Defining Lines on Standards of 

 Length. f — The delicacy of the method of measurement in wave-lengths 

 described in the preceding abstract calls for a corresponding refinement 

 in the engraved lines, which form the defining lines of the length of a 

 standard yard or metre or other line-measure bar. The defining lines on 



* Proc. Roy. Soc, Series A,lxxxiii. (1909) pp. 79-80. t Tom. cit., p. 81. 



