236 PROCEEDINGS OF SOCIETIES. 
Pleurosigma angulatum (Nav. stri- 
gosa, Soll.) ...(diag.) = 0°05—(less than 5 VIIth-metres.) 
; esteiarit ...... (diag.) == 0:05—(Ditto.) 
af fasciola (Cerat. fas- 
ciola, Soll....(trans.) = 0°04 (4 VIIth-metres.) 
Navicula rhomboides ...... (trans.) = 0°03 (3 VIIth-metres.) 
Nitschia sigmoidea (Nav. sig- 
moided, Soll.) .........0+ (trans.) = 0°03 (Ditto.) 
It is of interest to compare these measures of test objects with 
the lengths of waves of light. 
The wave-lengths of violet, indigo, and blue rays, range from a 
little below 0°04 to 0:05 ; those of green and yellow rays between 
0:05 and 0:06 ; and orange and red rays from 0°06 to nearly 00°8 ; 
rays longer than 0:07 being, however, found only in light directly 
from the sun or other intense source. 
lt appears, then, that the foregoing test objects are as minute 
as the wave-lengths of visible light. Indeed, if the determina- 
tions can be depended on, the last two are even somewhat 
smaller than the shortest visible vibrations. This, if it can be 
established, is a very unexpected fact. The best way of testing 
it would perhaps be by one of the finer of Nobert’s scales. If 
any member of the Club have such a scale he may convert its 
measures into metrical measures, by allowing 2°256 millimetres 
(which is the same as 225°6 Vth-metres) for each line or twelfth 
part of the obsolete Paris inch made use of by Nobert. 
If, as appears to follow from theory, light of wave-lengths 
longer than the intervals between markings is inoperative, it would 
appear that it must be positively mischievous, by producing a 
haze of brightness through which the markings are to be dimly 
made out. Accordingly, in scrutinising strie separated by an 
interval of 0:07, such as those of Pleurosigma formosum, the full 
light of the lamp may perhaps be used with advantage ; but in 
examining such markings as those of P. hippocampus, with an 
interval of 0:06, it would appear that the red rays at least should 
be cut off; and that, when the object is so small as the markings 
of P. angulatum, ‘viz. 0°05 and under, none but blue, violet, and 
indigo rays shouid be permitted to pass. There seems to be 
some ground for hoping that by this treatment these difficult 
objects will become more manageable. Blue shades have some- 
times been used by microscopists, but, apparently, without 
knowing why or under what circumstances they are of service. 
Glasses coloured blue by cobalt have the disadvantage that they 
transmit red rays, and on this account the ammonio-sulphate of 
copper*, which allows only rays of high refrangibility to pass, will 
probably be found a much better absorbing medium. 
* This invaluable absorbing medium is prepared in a few minutes by dis- 
solving a little sulphate of copper in water, and adding liquid ammonia 
until the white bulky precipitate, which first forms, is redissolved. Ifa 
dilute solution be wanted, water is to be added, which causes a dark preci- 
pitate, and then ammonia, until the precipitate is nearly dissolved. The 
concentrated solution in a watch glass, or a dilute solution in a tube closed 
by plate glass ends, will be found to answer. 
