ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 133 



several degrees finer than I have been able to detect under the 

 Microscope. About four years since I sent to Prof. J. Edwards 

 Smith a ruled plate with a statement of the number of bands, accom- 

 panied with a description of the same. Soon after I received a lettor 

 from Prof. Smith, saying there must be some mistake in the descrip- 

 tion, as he was unable to find two of the bands. I replied that the 

 bands were certainly ruled, and that I thought I could convince him 

 of that fact. I therefore requested him to re-esamiae the plate with 

 the greatest care, and if he was still unable to find the bands to return 

 the plate to me. After a vain endeavour to discover them the plate 

 was sent to me. I removed the cover, filled the lines with graphite, 

 remounted the slide, and returned it to Prof. Smith. Not only had 

 the invisible bands become visible, but the separate lines, with an 

 interlinear space of 1/80,000 in., were easily seen. Now when 

 Prof. J. Edwards Smith, an acknowledged expert in the manipulation 

 of the Microscope, is unable to find lines which are really in the centre 

 of the field of the Microscope, I suspect that other observers may find 

 a similar difficulty. Among the plates presented is one series which 

 were ruled to illustrate the possibility of producing lines which really 

 exist, but which are invisible under the Microscope. On one plate 

 there are two sets of lines, one set on the slide and the other on the 

 under side of the cover. Between the bands, 10,000 and 24,000 to the 

 inch, the entire intervening space is filled with a continuous series of 

 bands, 24,000 to the inch. I have not been able to see the lines of the 

 last band. In another plate there are a series of bands containing 

 twenty-one lines each, the entire linear space being 1/2000 in. 

 The first eleven lines are ruled with a forward motion of the 

 diamond, and the second ten lines are ruled with a backward motion. 

 Tiie last two bands are preceded by heavy finding lines. Each of the 

 last three bands is followed by bands 24,000 to the inch. I think 

 it will be found difficult to see the lines of the last two bands under 

 any illumination at present in use, and yet I am confident that the 

 lines exist. I found my belief upon two bits of evidence : First, the 

 pressure of the diamond upon the glass was sufficient to produce the 

 lines. With considerable less pressure there would still have been a 

 constant contact between the diamond and the glass. Second, I saw 

 them ruled through the sense of bearing. When a diamond does its 

 very best work it produces a sharp, singing tone, which is audible at 

 a distance as great as twelve inches. This singing tone I distinctly 

 heard for every line ruled. It is even more marked in ruling the 

 finest lines than in coarse ones. I have two singing diamonds, or 

 rather two diamonds with singing crystals, and these two are the 

 ones with which I have done my best work. 



The argument against the visibility of single-ruled lines which 

 cannot be seen with the present means at command, even if within 

 the limits of possibility, considered in a physiological sense, is in 

 one respect a sufficient answer to the evidence offered in favour 

 of their existence. This evidence, while not exactly negative in 

 its character, is yet not sufficiently conclusive to be regarded as 

 coming under the head of proof through the medium by which the 



