V 



MICROSCOPICAL EXAMINATION. 271 



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 

 to secure an absolutely dark field, this can be done with a good 

 instrument and a cardboard tube over the slide with a near 

 approach to completeness. Any marked illumination of the 

 field when the Nicol prisms are crossed will greatly impair the 

 delicacy of the test. 



Microscopical Examination after Treatment with Sol- 

 vents. A. Zega has suggested the following process : The 

 sample is melted and filtered into a test-tube, which is kept for 

 two minutes in a boiling water-bath. By means of a hot pipette 

 1 c.c. is measured into a 50 c.c. stoppered tube containing 20 c.c. 

 of a mixture of 6 parts of ether, 4 of alcohol, and 1 of glacial 

 acetic acid. The whole is shaken well, and allowed to cool in 

 water at 15 or 18 C. Pure butter remains clear, and only gives 

 a slight deposit after standing one or one and a half hours. Mar- 

 garine shows a deposit in one or two minutes, and in ten minutes 

 yields a copious precipitate. Mixtures of butter with 10 per 

 cent, of margarine begin to separate in about fifteen minutes. 

 As soon as a few solid particles have fallen, they are withdrawn 

 and examined under the microscope. Genuine butter appears 

 in long, very narrow crystalline rods, often pointed at the ends, 

 sometimes bent, and usually joined together centrally into more 

 or less symmetrical open stars. Margarine crystals consist of 

 bundles of minute needles packed closely together into circles, 

 sheaves, or dumb-bell-like masses. 



