144 
Notes. 
of wanting to see the finest structure ; the reason for this being, of 
course, that the blue rays diffracted from very fine structure may be 
just grasped by the objective, whilst the corresponding red ones may 
be just outside its cone. 
“ In some previous papers I have mentioned that discs for use 
above the objective might he made by cementing two cover-glasses 
together by Canada balsam, filmed surface inwards. Further trials 
have shown, however, that such discs are inferior in use to single 
cover-glasses covered with collodion, for anything higher than 1/3 in. 
objective, because the thickness of the double glass affects the correc- 
tion of the lens too much, even when fitted with correction collar. 
With a thin single glass, the corrections are not appreciably disturbed, 
and can be compensated for entirely by the correction collar and tube- 
length. 
“.Reverting now to the employment of colour-discs in the con- 
denser, I should like to show you my latest illuminator (fig. 29). You 
will observe it consists of a box, open at the ends, fitted under the con- 
denser, in which there are a number of metal carriers, which can be 
pulled out or pushed in quite independently of one another by means 
of little handles. So that they may slide freely, each carrier is sepa- 
rated from the next by a sheet of celluloid. Each carrier has two 
circular apertures, the one being fitted with a colour-disc or other 
stop, the other one being left free. The kind of stop is indicated on 
the handle. The openings in the carriers are so arranged that when 
the apparatus is closed all the free openiugs coincide, so that illumina- 
tion can be effected in the ordinary way. When any other illumina- 
tion is required, it is only necessary to pull out the particular stop or 
combination of stops, each stop being in accurate position when pulled 
out as far as it will go. The special thing about the particular 
illuminator I have here, is that the edges of the carriers are turned 
over, so that the gelatin or other stop can be slid into them freely, 
and therefore easily centered. 
“ Though the illuminator is bulky in the hand, it packs away 
under the stage of the Microscope, so as to be out of the way, and in 
use it is very convenient for rapidly comparing the effects of different 
stops. 
“ A good large source of light is the best when low-power or the 
composition method of illumination is resorted to. This may be 
obtained by using the bull’s-eye in the usual way, or by placing a 
piece of ground glass immediately in front of the luminant, and 
dispensing with the bull’s-eye. Personally I always use a Welsbach 
light, with a piece of ground glass just in front. In this way the 
source of light, besides having considerable extent, is a truly plane 
surface, and this I think is a great advantage over any source of light 
of irregular shape such as a lamp flame. Indeed, if 1 may say so, I 
believe insufficient attention has been given to using a plane surface 
of light in general microscopic work ; which is curious enough, con- 
