128 Transactions of the Royal Microscopical Society. 
surfaces, hollow or solid, and of different densities. The effects 
produced by the reflection of light from objects in numerous 
modes. The varied and deceptive appearances brought about by 
the interference of light, a subject which in England, at any rate, 
is in its relation to the microscope almost untouched. The effects 
produced by double refraction and polarization. The variations, in 
differing circumstances, in the colours of. bodies. The large range 
of phenomena involved in the appearances presented by the same 
object with different kinds of illumination, or in media of different 
refractive powers. The determination of the indices of refraction 
and dispersive powers of objects (matters which materially aid in 
the determination of their true nature). The discrimination of 
holes from mere depressions, and other variations of structure. 
A few clearly settled generalizations would enable a large group 
of phenomena to be resolved. 
As an example of the way in which what is only one small 
item of this subject may he treated, I may refer to the discussion 
by Nageli and Schwendener of the interpretation of the image of 
a hollow cylinder, a typical form of microscopic object. 
They discuss the theory of the formation of the image in four 
groups : — 
1st. The peripheral rays which traverse the walls of the 
cylinder without entering the cavity. 
2nd. The peripheral rays which strike the internal surface of 
the cylinder very obliquely, and are there reflected. 
3rd. The rays which penetrate into the cavity of the cylinder 
and are reflected on the walls, then arriving at the objective after 
having undergone two refractions (that is in all, four refractions 
and one reflection). 
4th. The rays which traverse the cavity of the cylinder in a 
straight line and undergo a quadruple refraction. 
Thus is shown on theoretical principles the appearances which 
such objects will present in different media, or in the other varying 
conditions in which they may be placed. 
I have disclaimed the necessity of having to prove the practical 
use of such investigations ; but “ without prejudice ” (as the lawyers 
put it) to that position, I may ask if it can be doubted that such 
generalizations would be of the greatest assistance to other observers, 
and the practical results important and useful ? 
There are numerous instances of the way in which the appre- 
ciation of purely optical or physical principles has served in the 
past to elucidate biological questions, and may serve to elucidate 
others in the future. I remember having read (though I have 
been unable to verify the passage) that Harvey himself attributed 
his discovery of the circulation of the blood to the investigations 
he had previously made on the pressure of liquids in tubes. 
