MICROSCOPIC EXAIIIXATIOX UXDER POLARISED LIGHT. 339 



with a good emulsor, separating the cream, and churning this 

 with ordinary cream, behaves as butter, and Pizzi has succeeded 

 in adding 30 per cent, of foreign fat to butter in this way without 

 being able to distinguish it. Finally, rancid butter, and butter 

 which has been at once churned from pasteurised cream at a 

 low temperature, may sometimes give an appearance resembling 

 margarine. Butter prepared from clotted cream shows many 

 crystalline particles (Fig. 46). 



It is apparent that this test must be used with reservation, 

 but it is without doubt of use as corroborative evidence in cases 

 where other analytical data are not absolutely conclusive. 



The method is carried out as follows : — The outer portions of 

 a piece of butter are removed, and a piece about the size of a 

 pin's head is transferred from the freshly exposed surface to a 

 clean microscope slide. A cover glass is placed on the top, and 



Fig. 46. — Butter under Polarised Light. 



the butter spread out by gentle pressure on the upper surface of 

 the cover. The slide is placed on the stage of a microscope fitted 

 with crossed Nicol prisms, and exammed with a 1-inch objective 

 or higher power. To exclude light from the upper surface a 

 blackened cardboard tube may be placed over the slide in such a 

 manner that the objective dips into it, and the light falling on 

 the upper portion of the slide is cut off. When piu-e butter is 

 examined the field is uniformly dark, and only with the greatest 

 difficulty can any structure be distinguished. ^Mien margarine 

 is present certain portions of the field have a bright appearance, 

 and indistinct crystalline forms can be made out. If any distinct 

 and bright crystals are seen, the Nicol prisms should be turned 

 parallel, and the slide examined in that spot in order to see 

 whether salt or other crystalline matter is present ; there is not 



