AIR BENEATH THE MICROSCOPE. 
163 
be very fairly contended, by those who are opposed to the 
views laid down by the men who have written upon the subject, 
that from the use of inferior powers a whole world of animal 
life was cut off from their gaze ; but of this more hereafter. 
We have now to examine the question, what does the air con- 
tain ? And of course this is a difficult problem, from the very 
different nature of the questions that may be put. For instance, 
we must say in the first case, supposing that the reader under- 
stood that the air consists of 77 per cent, of nitrogen and 23 
per cent, of oxygen, with a certain faint proportion of carbonic 
acid, ozone, &c., &c., that three distinct questions present them- 
selves at the outset. 1st, Is there any difference in the results 
of examination of air as to the locality which is explored ? 
2nd, Is there any difference as to the time at which our ex- 
ploration is conducted? and 3rd, Is there any distinction to 
be made in regard to the time at which the atmosphere is ex- 
amined ? And these three questions may be replied to generally 
in the negative — of course, making every allowance for trifling 
distinctions dependent on differences of temperature, and upon 
the effects of heavy falls of rain. The question naturally arises to 
one who is inexperienced with the microscope, how are you to 
examine the air ? If you look through a microscope simply at 
the atmosphere you will certainly see nothing, no matter how 
high your power. Well, then, it is necessary, in a certain 
sense, to do what is equivalent to condensing the air ; that is to 
say, we must have some means of examining at one glance several 
thousand cubic feet of the atmosphere. And how is this done ? 
By means of a very simple instrument which has been termed 
the aeroscope (see fig.l, plate CIX). Of course it must not be sup- 
posed that this is our only means, for we have the dust on leaves, 
and the specimens collected by means of moist-glass slides. But 
for all practical purposes the aeroscope furnishes us with a 
handy, easy, and most satisfactory apparatus for the investiga- 
tion of almost any quantity of atmospheric air. And what is 
the aeroscope ? It is an instrument whose use is to expose a 
glass slide in such a manner that a current of atmospheric air 
shall pass over it with considerable rapidity, and in such a fashion 
as to leave nearly all of its abnormal contents upon the slide. 
The instrument which is figured in our plate was originally 
devised by Dr. Maddox, hon. F.R.M.S., of Southampton, a 
gentleman well known for his investigations into the microscopic 
inhabitants of the atmosphere. But it has been very slightly 
modified by Dr. D. Cunningham, of Calcutta,* whose researches 
on the microscopic inhabitants of the air have been the fullest 
* u Microscopic Examinations of Air.” By D. Douglas Cunningham, 
M.B., Surgeon H.M. Indian Medical Service. Published by Government, 
1874. 
