the Polariscope presented by the President. 69 
at right angles to, the polarizing plane of the analysing Nicol, 
the former component passing, and the latter being arrested. As 
the angle of rotation is increased, the proportion of the latter 
component increases, and more and more light is arrested, until 
when the planes of polarization of the polarizer and analyser are 
at right angles to one another, the light is almost completely 
stopped, and the field becomes quite dark. Continuing the 
rotation beyond the right angle, the light gradually reappears. 
If, now, when the planes are at right angles to one another, we 
place ketween the polarizer and the analyser a transparent 
substance which has the optical property of rotating the plane of 
polarization, the polarized light which passes through this sub- 
stance will have its plane of polarization so altered that a portion 
of it will be able to pass through the second or analysing prism, 
and will produce on the screen or in the microscope an image of 
the object, more or less bright, on a dark ground. A similar 
effect is produced if a doubly refracting substance is placed 
between the two prisms, the light being resolved into two com- 
ponents, one of which is able to pass. 
Frequently, especially when the objects exhibited under the 
polariscope are thin sections, the images appear brightly coloured, 
the colours changing to the complementary hue when the ana- 
lyser is rotated at right angles to its former position. These 
colours are due to what is called ‘‘interference,’’ like the colours 
of thin films, as of a soap-bubble, though the mechanism pro- 
ducing them is different. In the thin film the light which is 
reflected from the deeper side of the film has to take a longer 
path than that reflected from the surface, hence it lags slightly 
behind, and if it does so to just such an extent that the vibra- 
tions in the retarded ray are moving in exactly the opposite 
direction to the corresponding ray with which it coincides, those 
particular waves are neutralized. Hence, as white light is made 
up of an indefinite number of series of waves of different fre- 
quencies, each corresponding to a particular colour, if one of 
these series of waves is cancelled out, the residual light will not 
be white, but of the colour complementary to that which has 
been cancelled out. In the case of polarized light a similar 
effect of colour is produced, owing to the circumstance that the 
light which emerges from the analyser after having passed 
through a doubly refracting object is composed of two portions 
which have travelled at slightly different rates of velocity, hence 
one portion lags behind another sufficiently to allow the waves 
of length corresponding to a particular colour to cancel one 
another, and the light appears of the complementary colour. In 
either case it is only when the thickness of the reflecting or 
doubly refracting medium is very small that the paths of the 
rays coincide with sufficient exactness to produce the inter- 
