28 



THE AMERICAN MONTHLY 



[February, 



tions. No black cordiform attach- 

 ments are at all visible. But there 

 are some scores of the species giving 

 great differences in detail. The 

 spherular resolution of these scales 

 offers an interesting field of research. 

 The blackness of the spherical rings 

 shows the small aperture of illumi- 

 nating cone and objective, whilst the 

 ribs and membranes are beautiful 

 studies with the best glasses. 



Mr. Slack's colloid silica slides pre- 

 sent remarkable examples of minute 

 refracting spherules, of considerable 

 variety, developing marginal black 

 rings and focal disks more or less 

 bright. Less than i -90,000th, this 

 bright central disk is scarcely visible ; 

 but those of i -60.000th. or less, with 

 a high but exquisite power, reveal 

 the focal centre as well as the black 

 ring in a slightly lower plane. It is 

 interesting in these objects to select 

 silica beads of different sizes, repre- 

 senting those of well-known diatoms, 

 such as those of Formosum and an- 

 gulatum for optical comparisons. 

 When a brilliant white disk in dia= 

 toms also can be detected, it is gen- 

 erally accompanied, as before, by a 

 jet black marginal ring all round the 

 spherule ; and in brilliant spherules 

 1-40, 000th of an inch in diameter this 

 black ring has been frequently esti- 

 mated at i-6th of the disk, or 1-240,- 

 000th of an inch thick. 



This very ring plays so important 

 a part in the definition of diatoms, 

 cells, and molecules that I shall ask 

 leave to call it the spherule test ring, 

 or, shortly, the test ring ; for, if a 

 glass giving 800 diameters will not 

 show it in a minute spherule (1-90- 

 oooth) it cannot be rated as of the 

 finest quality. Li many experiments 

 described in these articles, its use and 

 appearance are of the highest value. 

 I have to thank Mr. Slack for the 

 following formula : — 



' The silica films which give the 

 cracks are made by allowing a solu- 

 tion of dialyzed silica to evaporate 

 upon a slide. The beaded silica films 

 are made by passing silica fluoride 



gas through glycerin and water — 4 or 

 5 glycerin and i water.' 



Black margins can also be seen in 

 fine hairs of exquisite precision of 

 definition. 



In dealing with such minute mat- 

 ters, the reduction of diftraction by 

 a pleasantly subdued light — pale blue 

 glass has several advantages — an ex- 

 ceedingly fine glass of quite modern 

 date (18S5) thus showed me, last 

 night, the extremity of a fine hair less 

 than a millionth of an inch in diame- 

 ter without any indistinctness, fog, or 

 diftraction. Too much pains cannot 

 be taken with adjusting the screw 

 collar, and in the selection of the ob- 

 ject. Generallv it should be chosen 

 as close to the covering glass as pos- 

 sible : the film of air introduces other- 

 wise insuperable aberrations which 

 no action of the screw collar can sur- 

 mount, unless it be mounted in Can- 

 ada balsam. And this is more pain- 

 fully seen with oil immersions and • 

 dry mounts tlian with water lenses, 

 and still more than with the dry ob- 

 jective. The point of the hair is 

 often more clearly seen if there be 

 some subjacent structure. 



I shall now beg to record some 

 rather amazing experiences with the 

 definition of hairs with the finest glass 

 I could recently obtain — a i-i2th oil 

 immersion. 



The advance of the accuracy and 

 power of the microscope is well 

 shown in the developed structure of 

 hairs. A favorite object figured in 

 antiquated books is the hair of the 

 Indian bat. Qiiekett represents it as 

 frilled with a kind of coronet of small 

 hairs, ringed at regular intervals, 

 leaving the intermediate transparent 

 quill exposed. 



The drawing now given was taken 

 by the use of a fine oil-immersion 

 i-i2th, and a large angle in the oil con- 

 denser. Instead of frilled hairs, 

 which are purelv imaginary, a beau- 

 tifully serrated cup, wuth concave 

 notches, is seen, and edges as black 

 as jet, ornamenting the whole of the 

 stem at equal intervals. After so 



