ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 505 



more delicate kinds are invisible unless sought for by special means of 

 illumination. In Canada balsam, refraction index 1*52, we have a 

 medium of refractive power about as much above that of diatoms as 

 water is below it, and which therefore gives much the same difficulty. 

 Styrax, refraction index 1 ' 58, is markedly superior to Canada balsam 

 . . . Monobromnaphthaline, a liquid with a refraction index of IM16, 

 is very convenient for temporary use . . . and it is stated that in a few 

 months mounts made with it lose the advantage of its high refractive 

 power. . . . Piperine, with a refraction index of 1 ' 68 , . . serves 

 excellently for temporary purposes, as when the preparation is not 

 wanted for more than a few months. . . . 



" Canada balsam, styrax, and piperine allow of full advantage being 

 taken of homogeneous immersion systems, and hence secure the 

 maximum resolving power, while with air (a " dry " mount) the angle 

 of illumination is limited to something under N. A: 1*0. But neither 

 of these three media differs so much from the refractive power of 

 diatom silica as air does, and therefore none of them equals air as a 

 medium for the visibility (or photographability) of the object. When 

 we approach a refraction index of 2-0 we surpass air in this matter. 

 Sulphur alone has approximately this index, and realgar (pure As. 2 S 2 ) 

 is stated to have an index of refraction of 2 ■ 45, so that such media give 

 us advantages that we can never approach by media of refractive powers 

 lower than that of diatom silica. . . . 



" Realgar is a substance of definite composition (As 2 S. 2 ) which 

 occurs as a mineral and can be prepared artificially, but when the word 

 " realgar " is used to describe a medium for mounting it appears to mean 

 no more than that arsenic and sulphur enter into its composition. . . . 

 We may, for example, prepare a substance that contains arsenic and 

 sulphur in definite proportions (1) by direct combination of the two 

 elements ; (2) by heating together arsenious oxide and sulphur : (3) by 

 heating together realgar (As. 2 S 2 ) and sulphur ; (4) by heating together 

 the precipitated yellow sulphide of arsenic, or the mineral orpiment 

 (As. 2 S 3 ) and sulphur ; and the only matters for consideration are con- 

 venience and the purity of the materials. Purity is the most important 

 matter, and it may be taken for granted that any tinge of brownness 

 is due to dirt. Attempts at purification make matters worse rather 

 than better . . . We see when the sun shines how full of motes the air 

 is, and . . . none of these preparations nor the sulphur used in making 

 them should be ground up or subjected to any avoidable friction . . . 

 and any preparation made with sulphur that has been ground is 

 obnoxiously dirty, though the sulphur may originally have been pure. 



€ " In working with preparations such as we are speaking of, one 

 always gets some small fragments, and if the preparation has proved 

 good and clean, there is a temptation to melt up these small fragments 

 so as to get a piece of useful size. But it is useless, for the mere 

 manipulation of them seems to gather these aerial motes, and the stuff 

 is brown after refusion. 



" Purification of Materials. — I obtain the sulphur in a commercially 

 pure condition and distil it twice. I have distilled the same portion 

 seven or eight times successively, but there always remain in the retort 



