A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 17 



glass itself acting as a condenser, and a sharp image of the electric lamp filament 

 being thus projected, as above stated, across the field of the instrument. 



The electric lamps are only intended for preliminary work, and in actual measure- 

 ments each of them is replaced by an image, of the same shape and size as the little 

 straight filament, of a distant powerful source of light, a Nernst lamp. This avoids 

 all possibility of any thermal effect due to the proximity of the lamps to the bars. 

 A small total-reflection prism with rectilinear stop replaces the little lamp, and parallel 

 rays from an adjustable condenser in front of the distant Nernst lamp, filtered and 

 cooled by passage through an elongated cell containing copper acetate solution, are 

 directed upon the little prism, flooding the rectilinear aperture of the stop with 

 brilliant greenish-blue light, the best of all for efficient resolution of the Grayson 

 rulings, and free from all heat rays. 



The M -and- Plane Beds. 



As regards the V-and-plane beds, /* and t referred to under (2) and (5), they are 

 similar in all respects, except that the upper one is 2} inches wider than the lower 

 one, being continued to this extent backwards, as there is ample space on the top of 

 the stone block-support, while on the step below the width is limited. Each bed has 

 channelled in it from above, between the V and the plane, a _L-shaped groove, C, for 

 the purposes of the locking arrangements of the various sliders which have to rnove 

 over the two respective beds. The sections of the two beds will be clear from figs. 3 

 and 4. Very special care has been taken with the true planing of the sides 

 of the V, and of the surface of the plane in each case, as it is all-important for 

 interference wave-length work that these should attain the highest refinement of 

 accuracy. The final stage of this planing and the final polishing was done in situ on 

 the stone foundations after the best skill had previously been employed on them at 

 Messrs. Troughton and Simms' Works at Charlton. The castings were made under 

 direct supervision at West Bromwich, by Messrs. Kenrick, and were particularly 

 good ones, especially as regards homogeneity, and both were cast from the same melt. 

 Four highly concordant analyses proving this point most satisfactorily, of borings and 

 planings taken from the different parts of each bed, were made by Mr. C. Hobday at 

 Goldsmith's Hall, by kind permission of Sir Walter Prideaux, to whom sincere thanks 

 are due. 



The Microscope Sliding Blocks and their Fine Screw. 



A few further details of the two steel blocks which slide on the upper bed, and the 

 microscope-carrying slabs whose fine movement over these blocks is the critical point 

 of the whole instrument, will doubtless prove interesting, as they involve several 

 novel features. Each block (o, figs. 3 and 4) slides with V-and-plane contact over 

 the 6|--feet bed, h, and may be fixed at any position by a novel method of locking a 



VOL. COX. A. D 



