Scales of Insects and Fish. 40 



all around is the bright yellow background. As the 

 prism revolves the coloured portions of the scale seem 

 to revolve and change colour. When the selenite 

 appears blue the letter V's are blue, the angle inside 

 emerald green, and the exterior spaces deep pink ; 

 and this rotation of colours continues till the prism 

 has gone all round. Now this is the kind of effect 

 which the polarizing apparatus exhibits. The black 

 cross is shown on many objects, and the bright colours 

 appear in many others, with and without the selenite. 

 It is an exhibition which causes a good deal of plea- 

 sure and surprise, and I have often enjoyed studying 

 its phenomena. Yet in general I scarcely like to 

 bring it forward, feeling that the circumstances of 

 illumination under which objects are shown with the 

 polarizing apparatus are so peculiar, that I could not 

 answer the question, " Is it true ? "* with the same 

 unreserved affirmative that I should employ in vouch- 

 ing for the appearance of objects shown in ordinary 

 light. It may be asked, " Could not the principle of 

 polarization be explained at the time the objects are 

 shown, so that observers might take an intelligent 

 interest in the subject ? " To this I would answer 

 that some very deep principles of optics are involved 

 in the matter, and that the full explanation requires 

 a good deal of time, and would be unsuited alike to a 

 popular exhibition of the microscope, or to the plan 

 of a work like the present. 



A few words might however be said to give a 

 general idea of the effect of polarization, and the 



* See anecdote in the Preface. 



4 



