ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 241 



direction in which it is to travel. This is contrary to what one would 

 expect, comparing the action of a diamond with a steel graver or other 

 cutting instrument for like purposes, but when it is remembered that 

 the faces, the junction of which form the cutting edge, wear more 

 rapidly than the edge itself, one sees the analogy no longer holds good. 

 In the setting and adjustment of the diamonds it is important to 

 remember that, in the case of test rulings at any rate, the lines after 

 being ruled must on no account be rubbed or polished, consequently, 

 the material removed must be deposited on one side or the other of the 

 groove formed, and this involves the utmost nicety of adjustment of the 

 cutting edge, and not infrequently is a considerable tax upon one's time 

 and patience. The finer the ruling, the greater is the importance to be 

 attached to this particular feature. The length of the cutting edge is 

 also of moment. The longer the edge within certain limits, soon 

 ascertained by experience, and providing it is perfectly straight, the 

 longer will it endure, but as depth and breadth of line are important 

 factors, too long an edge implies too great a pressure strain to produce a 

 line of given depth and width. The pressure upon the diamond to 

 produce a line of a certain depth and breadth, I apply, in the case of 

 micrometric rulings, by means of a spring controlled by a screw ; this 

 gives good results up to a rate of 20,000 lines per inch, but beyond this 

 the friction involved is detrimental. The variation of pressure requisite 

 in test plate ruling is obtained by means of a series of weights ranging 

 from 20 grin, or more down to a fraction of 1 grm. 



In the matter of spacing, it is of the utmost importance that a 

 correct standard should be obtained as a basis for all micrometric 

 measurements. At the outset, I obtained copies of portions of the 

 standards in use at the Melbourne Observatory, both metrical and 

 English inch values. On carefully examining these I found a slight 

 discrepancy between the inch scale, as copied directly from the standard, 

 and the same values obtained by computation and ruling from the 

 metrical standard. As I had no means of determining which of the 

 two scales was more likely to be correct, I adopted the metrical scale as 

 it stood as my standard for metrical values, and the inch values, as 

 copied from the standard inch scale, as a standard for fractional values 

 of an inch. At a later date I submitted several micrometer rulings to 

 Mr. E. M. Nelson, a recognised authority upon all matters connected 

 with measurements of this character, with the result that it was found 

 that the ratio of inch to millimetre was, in the case of my inch rulings, 

 25 "8821 instead of 25 "89997 ; but as the metrical values proved to be 

 correct, in comparison with the best standards, I have since adopted this 

 scale as a basis for both systems. It may be of interest to know how I 

 determine that lines stated to be ruled, say, at the rate of 9o,000 per 

 inch, are really of that value. For this it is only necessary to adjust 

 the relationship of the wedge to the screw once for all, so that forty 

 revolutions of the latter give a movement equal to 0"02 in., in which 

 case one revolution will equal 0*0005 in. As the error in forty revolu- 

 tions can easily be brought within ^oixro m -> ^ ie error in ^ of this is a 

 negligible quantity. The screw-head being divided into 3G0 degrees 

 reading by a vernier to T V of a degree, 8 degrees of movement of the 

 screw-head advance the plate being ruled the ^g-innr part of an inch, 



