816 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
0 - 15 to 0*2 mm. from thicker blocks. The use of these cover-glasses 
also raises another difficulty, for, as above stated, no medium must 
intervene between object and objective with a lower refractive index than 
the number of the aperture, so that the object must be mounted in a 
medium whose refractive index has the required height. We do possess 
mounting media with refractive indices above 2*0 ; but the use of such 
media and the preparation of objects with them have their inconvenient 
side. 
They consist chiefly of arsenic and phosphorus compounds which 
are liable to give otf poisonous vapours or to explode during the prepa- 
ration of the object. Experiments with the system of aperture 1 * 60 
made with such mounting media have also shown that they are apt to 
attack the cover-glass so that the surface becomes rough and loses its 
transparency. Better results, no doubt, would have been obtained by 
the use of a different kind of glass, but in any case it is certain that 
the cover-glass of high refractive index will be more sensitive than the 
ordinary cover-glass, so that the choice of suitable mounting media will 
be considerably more limited than formerly. Altogether, then, the pre- 
paration of objects for these high apertures will be a much more difficult 
and costly process than with the apertures at present in use. 
Another difficulty arises when the object is of organic nature and is 
attacked by these highly refractive mounting media. A large class also 
of organic bodies requires to be placed in special media as like their 
natural surroundings as possible. Such media have refractive indices 
from 1 • 33 to 1*6 at the highest. This circumstance therefore sets a 
limit for such substances to any extension of the aperture, and in this 
case recourse must be had to the second method for increasing the capa- 
city of the. Microscope, which consists in diminishing A, the wave-length 
of the effective light. 
Now the absolute energy of the sun’s rays is different in different 
parts of the spectrum, and the sensitiveness of the eye varies for the 
different colours. The strength of impression of white daylight on 
the eye is therefore represented by a curve. The maximum point of this 
curve lies at A = 0‘55/x, so that from waves of this wave-length and 
those near to it the eye will receive by far the strongest impression, so 
much so that the partial images corresponding to the smaller and larger 
wave-lengths will be to a great extent rendered ineffective. But if these 
more energetic rays of wave-length 0 * 55 /x and those of greater wave- 
length be in any way excluded, and only rays of shorter wave-length 
admitted to the eye, then, under favourable circumstances — i. e. with a 
sufficiently intense source of light — the light of these short waves can be 
made to a certain extent effective. Thus it is well known what an aston- 
ishing increase there is in the resolving power of an objective when, either 
by the use of monochromatic light, or by the interposition of absorption 
glasses, a preparation is observed under pure blue illumination. Often 
a preparation which with ordinary illumination is beyond the limits of 
resolution, with monochromatic blue light, with the same objective, and 
under otherwise exactly the same conditions, is clearly resolved. In 
fact, the eye is sufficiently sensitive for the wave-length 0*44 /x to 
receive quite an intense impression when other light is excluded. A 
diminution of the effective wave-length from 0*55 to 0*44 /x, however, 
