for Christiansen's Experiment. 359 



faces of the glass sides of the bottle can be worked flat, while 

 the unavoidable irregularities of the inner surfaces are com- 

 pensated by the liquid, which, being adjusted to have the 

 same index as the powder, will have also the same index as 

 the glass of the bottle. 



The bottles that I have used* are about 3 inches high, 

 1^ inch wide, and about f inch thick, outside measurement. 

 The outer surfaces are worked (like plate glass), and not 

 merely flattened upon a wheel, as is usual with ordinary per- 

 fume bottles. For my earlier trials I was provided with a 

 piece of flint glass from the same pot as the bottles ; but 

 although the experiment succeeded well enough as regards 

 the elimination of the internal irregularities of the walls, the 

 glass-powder itself did not behave as well as I had seen plate- 

 glass powder do. It appeared ultimately that the flint was 

 not sufficiently homogeneous for the purpose, and another 

 specimen of flint was also a partial failure, from the same 

 cause ; but a sample of optical flint, kindly supplied to me by 

 Dr. Hopkinson, gave excellent results. 



It is more important that the powder should be homo- 

 geneous in itself than that it should correspond very accu- 

 rately with the glass of the bottle. For ordinary purposes 

 plate-glass powder (all, of course, from one piece) may be 

 used in a bottle of soda-glass, or even of ordinary low flint. 

 In preparing the powder great care is required to exclude 

 dirt. With respect to the coarser grades there is no great 

 difficulty, but the finer powder is apt to be contaminated with 

 the substance of the mortar. I prefer to use one of iron, so 

 that a magnet will remove the foreign matter. The elimination 

 of fine dust is also facilitated by a blast of wind from bellows. 

 In order to get good definition it is necessary not only that 

 the powder be homogeneous, but that the temperature be 

 uniform ; for, as Christiansen has shown, the transmitted ray 

 rises rapidly in refrangibility with temperature. In order to 

 secure homogeneity it is sometimes necessary to shake up the 

 preparation, which (to prevent the formation of air-bubbles) 

 is best done with a rather gentle motion while the bottle is 

 held nearly horizontal, The proportion of liquids necessary 

 varies with the temperature and with the kind of glass. Flint 

 will require a higher proportion of bisulphide of carbon than 

 plate-glas-s. If the transmitted light is too high in the spec- 

 trum, add more bisulphide of carbon ; if too low, more 

 benzole. 



The experimenter, working in a room lighted in the ordi- 

 nary manner, will be disposed at first to underrate the degree 

 * Supplied by Messrs. Harvey and Peak, Beak Street, Regent Street. 



