126 Transactions of the Boyal Microscopical Society. 
It would perhaps be only imitating the old quarrel of the 
logicians to discuss whether there can be any science of microscopy 
outside these limits ; but if it be otherwise, the boundaries of the 
science must be as extensive as the world itself, the elephants and 
palms having as good a claim in that case to be considered “ micro- 
scopic objects ” as the rotifers and the diatoms. 
I need not dwell on the first branch further than to point out 
that our text-books give the uninstructed reader the impression 
that the microscope stands in point of principle on no higher 
level than the telescope. This is far too low an estimate, and it 
would he truer to say that the microscope stands to the telescope 
at as great a relative distance as the chronometer stands to the 
sun-dial. Without having before them the means of comparison, 
it will, I know, be difficult, if not impossible, to convince the Fellows 
how much there is in this respect of which we know absolutely 
nothing: when the comparison has been made, the absurdity 
of the chapter in our books which professes to deal with “ the 
optical principles of the microscope” will be fully appreciated. 
Whilst as M. Eobin, the French histologist and microscopist, 
says, it is absolutely necessary to be familiar with all that con- 
cerns dioptrics when one has to make use of an instrument whose 
invention has been inspired by the discoveries made in this part 
of physics,” I do not know a book that even attempts to instruct 
the microscopist as to the course of the rays in passing through the 
ordinary objective of a compound microscope, or ventures upon 
anything beyond the barest mention of diffraction — the most 
common of all microscopical phenomena, and most intimately con- 
cerning not only the theory but the practical working of the 
microscope. 
At the present time there is nothing extant which constitutes 
even a commencement of a systematic theoretical treatment of the 
subject of illumination, yet, being purely optical, it is obviously 
capable of being so treated, and great practical advantages would 
undoubtedly follow from the development of an exact theory on 
the subject. In nothing has the ingenuity of microscopists been 
more exercised than in the invention of novel modes of illumination 
for lined objects ; but however clearly these various appliances may 
bring out particular appearances, there is good reason to believe 
that in the majority of cases they are illusions, originating in the 
character of the illumination employed, and that all possible 
methods of illumination may he reduced from the fifty or more 
kinds now existing to less than half a dozen at the most. 
With regard to the second branch, the conditions under 
which microscopic vision takes place necessitate (at any rate, 
in our present stage of experience) a more or less laborious 
reasoning process before we can recognize what it is we really see. 
