716 Dr. Gr. W. 0. Kaye on a Method of 



Accordingly, quite close up to one edge of the grating 

 were ruled by the Laboratory dividing-engine short fine 

 equidistant reference-lines each about £ mm. long and 

 at such a distance apart as to include about 100 grating- 

 lines . 



Each fifth reference-line was a trifle longer than its 

 neighbours, and each tenth line longer still. Every fiftieth 

 line was distinguished by the addition of an appropriate 

 number of fine dots, so that afterwards there was very little 

 trouble in picking up under the microscope any particular 

 reference-line required. 



The counting of the grating rulings was carried out with 

 the aid of the projection microscope. The grating was 

 mounted on the stage of the microscope and an image of the 

 graduated edge of the grating was thrown upon a screen : 

 with the magnification of 1200 employed the lines stood out 

 in sharp relief about 2 mm. a] tart on the screen. A 4 mm. 

 objective was used, and about lf)0 grating lines and two 

 reference-lines were in focus in the field of view. On the screen 

 was drawn a scale 1 of divisions of which the central 100 were 

 emphasized. The length of each scale-division was equal to 

 the distance apart of the lines in the projected image of the 

 grating. Thus by slight adjustment of the screen to one 

 side or the other, and so securing coincidence between the 

 lines of the scale and of the grating image, 100 (or so) lines 

 could be counted merely at a glance. There was no difficulty, 

 therefore, in noting the number of rulings (always near 100) 

 between each successive pair of reference-lines, the grating 

 being racked along each time on the stage of the microscope 

 by a convenient amount. 



Fig. 1 (PI. XV.) is a photograph with a magnification 

 of 450, and will give an idea of the appearance of the edge 

 of the grating together with two of the reference-lines. 



The method proved to be very expeditious. Without 

 interruption 1000 lines could be counted in 4 minutes; in a 

 2-J hours sitting 22,000 lines were enumerated without 

 fatigue. A good fraction of this time was taken up with 

 such things as attention to the illuminating arc of the 

 microscope, renewal of carbons, refoenssing, &c. 



It may perhaps not be without interest to add that in the 

 case of Mr. Gilford's grating, two independent countings 

 and a supplementary check counting agreed in giving a total 

 of 45,668 rulings. 



At 16°0 C. on the hydrogen scale, these rulings occupied 

 a length of 8*03618 cms., as the result of a comparison 



