340 BUTTER. 



much difficulty in distinguisting this owing to its great refrac- 

 tive power. The slide should be moved about to examine all 

 parts of it, as, in cases of small amounts of adulteration, the 

 margarine is not equally distributed throughout, and two or 

 more portions from different parts of the sample may be examined. 

 As a check, a selenite plate (a crystalline form of calcium 

 sulphate, which possesses the property of rotatory dispersion to 

 a large extent) is next placed under the slide, the microscope 

 focussed, and the sample again examined. In this case the 

 slide will be uniformly illuminated when the prisms are crossed, 

 but will appear coloured ; the colour depends on the thickness 

 of the selenite and the position of the Nicol prisms, but when 

 pure butter is examined the whole of the field appears of one 

 colour. When margarine is under observation certain parts of 

 the field are seen to be of a different colour. 



This modification is, when used by persons of absolutely 

 normal vision, quite as delicate as the examination without 

 selenite, but it cannot be generally recommended, as the per- 

 ception of colour is a sense in which many people — more than is 

 commonly supposed — are somewhat deficient, though not abso- 

 lutely colour blind. The usual colours which selenite plates are 

 constructed to give — red and green — are those which are least 

 easily distinguished by the majority of those who suffer from 

 weak colour perception. It is advisable, therefore, never to 

 omit the examination without a selenite plate. 



It is, of course, essential to employ a good microscope, as 

 any illumination of the slide, except by light which has passed 

 through the polariser, will prevent the extinction of the field 

 on crossing the Nicol prisms. Though it is impossible in practice 



