^ouraS. ^ISTi'm^] cf^nd the Structure of the Podura Scale, 83 
cannot fail to interpret the minute wonders of Creation to many 
intelligent admirers of Nature. 
P.S. — July 21. — In consequence of a question just put to me 
by one of the early Fellows of our Society, the Kev. Charles 
Pritchard, between whom and myself the microscope was in years 
long gone by a bond of union, I find it necessary to add a few ad- 
ditional remarks. The question is, " How does the jprism, as such, 
effect the work better than a good plane surface ? " Before answering 
the question I thought it better to examine the quantity and nature 
of the light which is reflected up the tube of the microscope from the 
left-hand plane face of the prism — an angle of the prism being 
towards the object on the stage — and the same Hght which passes 
through the same face of the prism and is totally reflected up the 
tube from the adjacent face or base of the prism, by turning the 
prism a little on its axis, the prism lying nearly under the vertical 
diameter of the stage. My report may " sound like a fable," but 
nevertheless the difference is marvellous. The whole light of the 
lamp totally reflected is not perceptibly altered either in nature or 
quantity, but the portion reflected from the face, and not entering 
the prism, is a 'purely polarized team ! As such I have used it in 
the examination of several suitable objects, passing it through sele- 
nite plates where necessary, and I prefer the results to any pre- 
viously obtained by the direct light of a common Nicol's prism. 
Salicine and crystals generally, as well as fine vessels in animal 
and vegetable tissues, are seen in almost stereoscopic relief, in con- 
sequence of the shadows which are readily thrown by a slight 
obliquity of the polarizing pencil. This obliquity may be extended 
to the bringing out the effect of polarized light even on a dark 
ground, and thus, as in the combination devised by Mr. Furze, 
heightening the solidity by the play of colours. 
The plane prism may be used in other instruments as a polarizer, 
but it is satisfactory to find that the prism microscope is independent 
of extra appliances for producing polarized light. 
I will only add, that when the sun itself is reflected from a sur- 
face of the prism, its disc being seen at the bottom of the tube, the 
phenomena of polarization, so easily exhibited, are brilliant in the 
extreme. The eye, also, is not fatigued by the brightness of this 
one component part of the sun's light ; but the whole light totally 
reflected from an inner face of the prism would be intolerable. The 
brightness of the polarized beam may, however, be diminished to 
any extent by simply placing small pieces of white linen of different 
thicknesses between the prism and the sun. 
The plane speculum of a Newtonian Telescope exhibits less 
polarization, both with daylight and lamplight, than the plane 
G 2 
