SPECIAL TESTS I 3 



limited to the non-cellular portions of such substances as jams and 

 jellies, indicating the use of fruit juices obtained from unripe 

 fruits. 



7. Microscopical Examination of Bacteria and Metals by 

 Direct Sunlight.' — Very minute quantities of certain minerals 

 as iron, copper, mercury, and a few others, can be detected in 

 liquids and semiliquids (in the form of metallic hydroxides) 

 when examined (on slide mounts) by means of direct sunlight. 

 All transmissible light must be cut off. 



Direct sunlight can also be used in making bacterial counts in 

 liquids, using the Thoma-Zeiss hemacytometer (Turck ruling). 

 The bacteria are readily recognizable on the dark background, 

 standing out far more clearly than in the usual examination by 

 transmitted light, because of the more pronounced color contrasts . 



8. Micro-gluten Test. — Mount a bit of the flour in water on a 

 slide, being careful not to use too much water. Cover with 

 cover glass and move cover glass to and fro a few times on the 

 mounted material. The gluten separates into stringy fragments 

 which may readily be seen under the low power of the compound 

 microscope. The use of a weak solution of carbol-fuchsin, 

 sofranin, or other stain, will bring out the gluten particles more 

 clearly. 



9. Hand Gluten Test. — Moisten wheat flour with water, making 

 it into a dough. Knead constantly and carefully under a slow 



' The optical principles of the ultra-microscope of Zsygmondy and Siedentopf 

 depend upon the use of direct sunlight (or other intense light) combined with an 

 absolutely dark field, with or without the use of a condenser, the rays of light being 

 directed upon the object to be examined approximately at right angles to the 

 optical axis of the compound microscope. 



The limits of vision with the ultra-microscope are approximately 0.003^, however, 

 solid particles (as of metallic colloids) of not more than 0.003/i in diameter show no 

 structure, they appear rather as points of light. 



The limits of vision with the ordinary microscope are, for air (white light) about 

 0.30/j, for homogeneous immersion (white light) about o.as^u, and for homogeneous 

 immersion when rays of shorter wave length than white light (as the blue spectrum) 

 are used, are about 0.15/1. 



