ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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This table shows that here, at least, there was an activity with regard 
to the condenser that was totally absent abroad. It must, moreover, be 
remembered that the list gives only types of condensers: Ross, for 
instance, made improvements on the Gillett, and Smith and Beck made 
numerous forms of condensers which have not been mentioned, simply 
because they were not typical. Messrs. Crouch and Collins made 
numerous condensers, mostly after the Webster type. So also, on the 
other side, Nachet and Hartnack fitted object-glasses as condensers, 
only to a much more limited extent. My impression is that, if statistical 
tables were available, it would be found that up to 1880 there were 
more condensers turned out by any one well-known English maker 
than by all the Continental firms put together. 
We now come to the use of the condenser, and the first question that 
arises is with regard to the nature of the source of light : Is daylight 
or lamplight to be used ? 
I find with low and medium powers, the condenser being centered 
to the optic axis, the plane mirror used, and a window-bar focused on 
the object, that daylight gives very good results, especially if a brightly 
illuminated white cloud is the illuminating source ; but when the white 
cloud has blown across the field, leaving only blue sky, the illumination 
becomes poor. My complaint against daylight illumination for low 
power work is that I believe it not only to be always changing, but also 
very injurious to the eyesight. When I began Microscopic work the 
white cloud was everything, but on account of the above-mentioned 
drawbacks I adopted artificial illumination. The most extraordinary 
ideas prevailed respecting artificial illumination. The history is as 
follows : — Brewster wrote a treatise on the Microscope in 1837, and in it 
explained his method of illumination. He was very keen on monochro- 
matic illumination ; this he obtained from some chemical substances 
flaming in a saucer, without any wick or chimney ; light from this was 
parallelized by a bull’s-eye formed by a Herschelian doublet, and this 
brought to a focus by another exactly similar lens. He is very particular 
to enforce that the image of a diaphragm placed between the source of 
illumination and the bull’s-eye should be focused on the object. This 
was in preachromatic days, and the kind of Microscope he experimented 
upon was the simple Microscope, the lenses being jewel singles, doublets, 
triplets, Coddingtons, which last were his own invention, &c. 
With such a source of illumination, unless his object had been in 
rays considerably condensed, he would not have seen anything at all. 
Be that as it may, the fact is that the rule of having the source of light 
in focus has been handed down by the text-books all along, only with 
this curious proviso, viz. that each author had his own particular 
directions for disregarding the rule. 
Taking Andrew Ross first, whose directions are considered so admir- 
able that Quekett says he will quote them at length, we find that after he 
has given instructions with regard to centering, he says that delicate 
objects are best seen by racking the condenser within, and objects 
having some little thickness without the focus. Further on he says 
that slight obliquity of the illumination subdues the glare attendant 
upon perfectly central and full illumination by lamplight ; he then goes 
on to say how this slight obliquity may be secured. The above words 
