72 SUMMARY OF CURRENT RESEARCHES RELATING TO 
or 130°, little more than half the light would be caught by the 
objective, and the light excluded is that which brings out the finest 
details in the standard image. This leads to — 
Propositions 3 and 4. 
When, of the light emitted by the object, only part is employed to form 
the microscopic image , then features may intrude themselves into the 
microscopic image which are not present in the standard image , and which 
do not represent anything upon the object ; and the partition of the light 
between the portions received by and excluded from the objective , will in 
general be different for different ivave-lengths ; and when the difference is 
marked, a colourless object will appear to be coloured in the Microscope. 
In connection with the use of the condenser we have — 
Proposition 5, 
The standard image is the outcome, partly of the features upon the 
object, and partly of the state of the light by which the object is illuminated. 
It may be improved by increasing the degree in which the first of those fac- 
tors, and by decreasing the degree in ivhich the second, contributes to produce, 
to modify , or to efface detail in the image. 
Proposition 6. 
Mounting the object in a medium of extra high refractive index will, 
cseteris paribus , increase the conspicuousness of the finer detail to be seen 
upon it. 
Proposition 7. 
If a microscopic object, mounted dry, is so close to the cover-glass that the 
chink of air between it and the cover-glass is less than the thickness of the 
Stokes's layer, then light from it can pass up through the cover-glass and the 
oil above it at angles both within and beyond the critical angle. 
The peculiar optical properties of a thin layer (Stokes’s layer) of a 
rare medium in contact with a denser were investigated by Sir George 
Stokes.* 
In the application of the above principles to the Microscope 'the 
author starts with the consideration of the illuminating apparatus. The 
source of light and the condenser are so far good as they enable the 
whole of the light of each wave-length which is brought to bear upon a 
point of the object to reach that point at each instant in the same phase- 
Efforts to attain this end should not be confined to the improvement of 
the condenser and to the cutting of thin sections. A further advance 
may be made by attending to the source of light, which ought to be 
confined to a thickness small compared with a wave-length. The 
appearance presented under the Microscope by the diatom Pinnularia 
nobilis is brought forward as an illustration of the confusion into which 
the light is thrown by traversing the substance of the object. Matters 
would be improved by mounting the diatom between two media, of which 
the under one has the same refractive index as silex, and the upper one- 
a refractive index as much as possible exceeding this. 
The successive stages of the transformation from the object to the 
image delineated on the retina (see fig. 2) are then carefully traced out. 
These stages are as follows : — 
A. Object A, the actual microscopic object, to which corresponds the 
* Stokes’s ‘ Collected Papers,’ ii. p. 5G. 
