A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 15 



focussed by the two microscopes, and a 4-inch bar still more readily. The coarse 

 adjustment is by rack and pinion, moving a very long dovetail slide, r, which is 

 attached directly to the tube. This can be readily clamped by the milled-headed 

 screw, v, when set to the approximate focus, so that no motion can take place at this 

 slide. The fine adjustment is by means of a micrometer screw, (f>, constructed with 

 special care, working a prismatic fitting sliding in a solid correspondingly prismatic- 

 shaped sleeve of great rigidity. This has been constructed on the well-known 

 principle of Messrs. Beck's solid-metal fine adjustment, in which the fitting has no 

 adjustable or spring pieces whatever, but is a long parallel prismatic fitting, solid 

 metal to metal. It is, however, made about double the size of an ordinary microscope 

 fine adjustment, and the result shows that the motion is absolutely free from back- 

 lash ; for the bands of the interferometer either do not move at all when the fine 

 adjustment is worked, or if they do very slightly, the movement is regular and can 

 be accurately calibrated, and is due to faulty setting of the mirror (j- A , which should 

 once for all be corrected. 



The microscopes carry double-motion cobweb micrometer eye-pieces, %, at the 

 upper end. Each micrometer carries one right-and-left spider-line and two front- 

 and-back parallel spider-lines. There are two drums, i//, each divided into 100 parts, 

 one of which moves one only of the front-and-back spider-lines, parallel to itself 

 and to the other front-and-back spider-line ; the other drum moves both simul- 

 taneously, when the distance apart has been regulated by the former. Two eye- 

 pieces, &>, magnifying respectively 8 and 1 5 times, are provided with each micrometer. 

 The object-glasses, O, for each microscope consist of a low-power for ordinary work, a 

 f-inch objective, magnifying 150 and 282 times with eye-pieces No. 1 and No. 2 

 respectively, and a Y^-inch dry lens, magnifying 1595 and 2990 times respectively 

 with the two eye-pieces, constructed with as long a working distance as is compatible 

 with the numerical aperture required to give the necessary resolving power for the 

 observation of the Grayson rulings, 40,000 to the inch, referred to in the Appendix. 

 The resolution required to observe lines 40,000 to the inch, which it must be 

 remembered correspond to single wave-length distances of red light, sufficiently well 

 to be able to set any one of them symmetrically between a pair of parallel spider- 

 lines is not less than double that required to actually resolve the lines, and the 

 result attained, which is eminently satisfactory as regards both illumination and 

 definition, may be taken as about the maximum possible with a dry lens involving 

 very perfect corrections of the various aberrations. Both the high-power objectives 

 have been specially calculated and corrected by Mr. Conrad Beck, whose valuable 

 original work on this subject is well known, for the specific purposes of this wave- 

 length comparator, in order to afford a specially clear definition of the Grayson 

 rulings without having to employ an immersion objective, which would be incon- 

 venient. The author is under great obligation to Mr. Beck for the truly excellent 

 definition attained with these wonderfully sharp and clear wave-length rulings. 



