242 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and so, proportionately, for other values up to 120,000 lines per inch, 

 the finest I have ruled which have so far been resolved. In passing, I 

 may state that the finest lines it has been possible to resolve or separate, 

 by means of the most perfect microscopical appliances hitherto con- 

 structed by the best makers, have not exceeded 120,000 per inch. 



I have yet said nothing concerning the glass most suitable for ruling 

 upon. Ordinarily the outer crust or surface of the glass as it leaves the 

 makers' hands is much too hard and brittle for the purpose, and speedily 

 ruins the hardest diamonds. This is especially so in the case of thin 

 unannealed- microscopical cover-glass, which it is essential to use for 

 many purposes. Hence it occurred to me that, it might be possible to so 

 modify and alter the surface of this glass by a process of annealing that 

 better results would be obtained. After some few trials I found that by 

 inclosing a carefully cleaned cover-glass in a metal capsule, and slowly 

 heating to a certain point, short of actual softening, and allowing the 

 cooling process to extend over as long a period as possible, the glass 

 proved to be both softer and tougher, and at the same time far less 

 liable to any alteration due to changes in temperature, or the relief of 

 certain surface strains inherent to the glass in its unannealed condition. 



I pass on now to a matter of equal importance with any hitherto 

 dealt with, viz., the preservation of the completed rulings. Ordinarily 

 in the case of micrometer rulings varying from 1 mm. to - 01 mm. all 

 that is necessary is to fill the lines with graphite, and mount the cover 

 on a slip with Canada balsam. But this method is not suited to the 

 finer rulings, or where it is desirable to preserve the lines without the 

 graphite filling, as in the case of test plates. Nor is it possible to pre- 

 serve them by attaching the cover-glass to a cell wall or ring of cement 

 or wax, as is frequently done with other microscopical preparations. I 

 myself tried every, or almost every, known cement and wax cell at all 

 suited to the purpose, and in every instance it was only a question of 

 time, probably a year or more, and the cover-glass became coated or 

 covered with minute crystals in some instances, or microscopical beads 

 of moisture in others, to such an extent as to detract greatly from the 

 beauty and perfection of the lines, and in some cases to partially oblite- 

 rate the finer bands altogether. It therefore remained for me to 

 endeavour to mount the ruled plates in a medium possessing a refractive 

 index differing from glass by an amount equal to the difference between 

 glass and air. Several such media existed, and had been used for other 

 purposes, but with only partial success. These were phosphorus, sulphur, 

 and realgar, or arsenic disulphide. The latter appeared to me the most 

 promising substance to work with, seeing it possesses a refractive index 

 equal to 2 "54;), but its use is attended with many difficulties, and I 

 worked with it for nearly a year with only partial success. I soon 

 abandoned all attempts to use it in a liquid form dissolved in the usual 

 solvent, bromine, which I found both uncertain and dangerous to use, 

 and turned my attention to the production of thin films, by sublima- 

 tion. With these I was more successful, and after a time was able to 

 produce exceedingly thin films, which have so far proved quite per- 

 manent. Some of the films here shown have been mounted over two 

 years, while those sent to London some little time ago withstood all the 



