ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1)5 
Another mistake is that direct light is more critical than indirect, 
which means, in other words, that illumination without a mirror is more 
critical thau illumination with a mirror. Presupposing the same con- 
ditions, viz. the same condenser with the same stop, the centering and 
focus being precise, the optical conditions must be identical and the 
result the same. The ground is entirely cut away from the one only 
thing which could possibly atfect the result — I allude to the loss of light 
by reflection at the mirror, by the fact that you have, with merely a 1/2- 
in. paraffin wick, more light than you know what to do with. So much 
is this the case that in my own practice I am in the habit of using a 
double cobalt pot-glass screen to reduce the intensity. 
1 am aware that direct illumination is a most convenient and time- 
saving method, especially when the instrument is well tucked up on its 
trunnions, but that it makes any perceptible difference in the criticalness 
of the image I am not prepared to admit. 
With regard to mirrors, a good deal of misapprehension exists. It 
matters little whether the mirror be dusty or scratched, or the silver in 
bad condition ; the only effect these will have will be to cause a little 
less light to fall on the back lens of the condenser, a matter supremely 
unimportant. An old scratched dull mirror will yield as critical an 
image as the finest worked up silver on glass Newtonian flat. 
The three things that are of paramount importance are the direction 
of the light, the angle of the cone, and the spherical aberration of the 
condenser. Mirrors which yield secondary reflections are to be avoided, 
but if they can be turned round in their cells the secondary images can 
be easily eliminated. 
Having touched upon the errors in the use of the substage condenser, 
let me say a few words with a view of clearing up some strange notions 
that are held with regard to its office. The original prevailing idea 
with regard to the office of a substage condenser was, I believe, in the 
first instance, that of a contrivance by which more light could be 
secured ; afterwards it became chiefly important as an oblique illumi- 
nator ; but its true function as that of a cone-producer was not generally 
recognized. As this view of mine will probably be met by the criticism 
that in the text-books, both ancient and modern, we read “ that the 
condenser must be accurately focused,” that the use of the diaphragm 
is for the purpose of contracting the cone of illumination ” (many similar 
passages might be quoted), I nevertheless contend that there are other 
passages which conclusively prove that the writers were ignorant of the 
true function of the condenser. 
The following is an example : — “ If the cone of rays should come to 
a focus in the object, the field is not unlikely to be crossed (in the 
daytime) by the images of window-bars or chimneys, or (at night) the 
form of the lamp-flame may be distinguished upon it ; the former must 
be got rid of by a slight change in the inclination of the mirror ; and if 
the latter cannot be dissipated in the same way, the lamp should be 
brought a little nearer.” 
This passage proves that the end-all and be-all in the writer’s mind 
was the agreeableness of the illumination ; when the glare of the lamp- 
flame becomes unpleasant, the cone may go to the wall. 
If the importance of the cone had been paramount in the mind of 
