WM. WEBB ON " NOBERt's TESTS." 159 



the incisions in the fine bands, scarcely any one incision having its 

 two sides of equal length in a direction from its lowest part to the 

 apices of the ridge on each side of the incision. I say apex be- 

 cause there is no other space dividing the incisions. And these 

 apices are of necessity irregular, because however rigid, however 

 perfect, however true may be the instrument, however capable to a 

 dead certainty may be the projection to the one 200-thousandths 

 of an inch, the very nature of the material worked upon, with the 

 two facts that the diamond has bevelled sides and the incision has 

 bevelled sides also, create a tendency to elasticity in the machinery 

 and materials, and as an evitable result the inequality of the ridges, 

 which can only be revealed by this or some analogous test. 



A little familiarity with the phenomena of the black lines pre- 

 pares one to consider what must be the effect of the refracted ray 

 from the long unbroken side of the outside cut crossing not only 

 the refracted ray from the short side of the cut, but over the first 

 apex and across the two rays from the two sides of the next inci- 

 sion. This complication of the phenomena has produced such a 

 confusion of aerial polarised black lines of light as to embarrass the 

 minds of some gentlemen, and driven them to resort to a declaration 

 of " spectral lines " without giving the slightest hint of their 

 source, and, apparently, wholly unconscious of the remarkable fact 

 that the so-called spectral lines can never interfere with the exami- 

 nation of the incisions, if they were all equal in depth, inasmuch as 

 the depth of the equal incisions and the spectral lines can never 

 with high powers be in focus at the same time as in the equal in- 

 cisions of the coarse bands where no one of the separate appearances, 

 whether of apparent hole throu-h the substance, or of black line, 

 or of depth, are visible under high powers at the same moment as 

 any one of the other appearances. I am not aware that the expres- 

 sion " spectral lines " has ever been applied to the coarser bands, 

 which may possibly arise from the fact of the operator failing to 

 recognise the dark beauties when arrayed exactly alike and with 

 naught else than their own aerial presence visible at the same time. 

 The production of the irregular polarised black lines I respectfully 

 suggest is an incontrovertible proof that the diagram numbered 6, 

 with its incisions having unequal sides, and its ridges of unequal 

 heighth, is a correct representation of a vertical section of the fine 

 bands, and the fact of Mr. Slack, after patient skilled labour, de- 

 spairing of being able to obtain a definition of colloid silica beceiuse 



