ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 127 



of 32 and 62 degrees Fahrenheit. The subdivision of these units into 

 aliquot parts — the yard into inches and the metre into centimetres — ■ 

 has been so far completed that any errors which may remain will not 

 affect the microscopical unit sought. With regard to the exact sub- 

 division of these units, I can only report progress. 



Notwithstanding this abandonment of attempts to produce test- 

 bands of the Nobert pattern, I have recently taken up the subject 

 again, somewhat with the view of testing the claim of Mr. Fasoldt 

 that he has succeeded in ruling lines one million to the inch, and 

 especially by the claim that the existence of a spectrum in the bands 

 is an evidence of the reality of the separate lines. The latter claim 

 does not appear to be well founded. Aside from being at variance 

 with theory, it can easily be disproved experimentally. 



Before proceeding further with this investigation, I beg to refer 

 to a theory proposed by the writer in a paper presented to the American 

 Academy of Arts and Sciences, in relation to the method which Nobert 

 may possibly have emj)loyed in the production of his test-plates. 

 Briefly stated, this theory is that the lines composing Nobert's bands 

 are produced by a single crystal of the ruling diamond, whose 

 ruling qualities improve with use. In the light of subsequent exj)eri- 

 ence this theory may be stated in the following way : When a diamond 

 is ground to a knife-edge, this edge is still made up of separate 

 crystals, though we may not be able to see them, and a perfect line is 

 obtained only when the ruling is done by a single crystal. When a 

 good knife-edge has been obtained the preparation for ruling consists 

 in finding a good crystal. Occasionally excellent ruling crystals are 

 obtained by splitting a diamond in the direction of one or more of the 

 twenty-four cleavage planes which are found in a perfectly formed 

 crystal. A ruling point formed in this way is, however, very 

 easily broken, and soon wears out. Experience has shown that the 

 best results are obtained by choosing a crystal having one glazed 

 surface and splitting off the opposite face. By grinding this split 

 face, a knife-edge is formed against the natural face of the diamond, 

 which will remain in good condition for a long time. When a ruling 

 crystal has been found which will produce moderately heavy lines of 

 the finest quality, it is at first generally too sharp for ruling lines finer 

 than 20,000 or 30,000 to the inch, even with the lightest possible 

 pressure of the surface of the glass. But gradually the edges of this cut- 

 ting crystal wear away by use until at last this particular crystal takes 

 the form of a true knife-edge, which is parallel with the line of motion 

 of the ruling slide. In other words, when a diamond has been so 

 adjusted as to yield lines of the best character its ruling qualities im- 

 prove with use. If Nobert had any so-called ' secret,' I believe this 

 to have been its substance. 



The problem of fine ruling consists of two parts — first, in tracing 

 lines of varying degrees of fineness ; and, second, in making the inter- 

 linear spaces equal. The latter part of the problem is purely 

 mechanical, and presents no difficulties which cannot be overcome by 

 mechanical skill. 



It will be the aim of the present paper to describe the more 



