Z PRESIDENTIAL ADDRESS. 



be possible to give more than the barest outline of the subject ; 

 but I trust that every one of my readers will derive a fairly clear 

 notion of how it is that we see certain appearances with a 

 polariscope. 



Every good microscope for general use is now provided with a 

 polariscope, so that the pretty colours in crystals, the crosses on 

 starch grains, the appearances in a section of rhinoceros horn and 

 the colours seen in plaited horse-hair under the polariscope, both 

 with and without selenite, are well kno-wn. But life is too short 

 for every microscopist to become at the same time a physical 

 optician, a botanist, a chemist, a geologist, a physiologist, an ento- 

 mologist, a bacteriologist, and a diatomist ; and my purpose now 

 is to try and explain a little about polarised light to those who have 

 not made physical optics their speciality. I shall not attempt to 

 explain any phenomena except those which relate to the construc- 

 tion of a polariscope and the appearances which it produces in 

 microscopic objects. 



What is polarised light? Mr. Spottiswoode, in his valuable 

 little book, says, by way of introduction : — " Light is said to be 

 polarised when it presents certain peculiarities, hereafter to be 

 described, which it is not generally found to possess ! " These 

 peculiarities he goes on to describe, but for our present purpose it 

 will be better to commence by asking the question, " What is 

 light ? " 



Sir Isaac Newton was the first who tried to answer this ques- 

 tion, but his " corpuscular theory," while accounting fairly well for 

 some of the simpler optical effects, presented such difficulties when 

 applied to polarisation and other phenomena, that it had to be 

 abandoned in favour of the wave theory, due to Huyghens and 

 Fresnel, and which is now universally accepted as the basis of 

 modern optics. 



The TJndulatory Theory.— According to this theory, light con- 

 sists in a series of vibrations, which are propagated through space 

 in the form of waves. In a light-wave the vibratory motion is in 

 a dirtciion perpendicular to that in which the light is traveUing. 

 To illustrate this, shake any part of a piece of string (stretched 

 horizontally between two fixed points) from side to side. You will 



