70 HOW TO WORK 



In Canada Balsam. In figs. 115 and 121 the crystals are shown 

 immersed in Canada balsam. The outline now appears as a sharp 

 well-defined line. In the case of the carbonate of lime a number of 

 narrow lines are seen radiating from the centre of each crystal to- 

 wards its circumference ; in fact the crystal really consists of a 

 congeries of minute acicular crystals. 



By Reflected Light. In figs. 116 and 118 the crystals are repre- 

 sented as they appear when examined by reflected light. The 

 globular form, and yellowish colour of the carbonate of lime, are very 

 distinctly seen, and the surfaces of the crystals generally seem slightly 

 rough, some appearing to be covered by minute elevations. 



By Polarised Light. In fig. 117 another preparation of the crystals 

 of carbonate of lime is seen under the influence of polarised light. 

 Each crystal exhibits a black cross which alters its position and 

 appearance as the analyser, p. 18, is rotated. 



The above important points might be illustrated by a vast number 

 of other substances. I cannot too strongly advise the observer to 

 subject various microscopical structures to examination in air, water, 

 and Canada balsam, and by direct or reflected, as well as under the 

 influence of transmitted light, and in some cases by polarised light. 



137. Of Air Bubbles, Oil Globules, ami Globules of Crystalline 

 Matter. It is of the utmost importance that the observer should 

 make himself familiar with the appearance of air bubbles and oil 

 globules as soon as possible, for he will often meet with them, and if 

 not acquainted with their characters he may make the most ridiculous 

 mistakes in describing specimens. 



Air Bubbles in water have a very wide dark outline : indeed, 

 small air bubbles appear like round black spots. This appearance is 

 very characteristic, and every observer ought to be thoroughly 

 familiar with it. Air bubbles of various sizes are represented in 

 pi. XIX, fig. 122. 



Oil Globules also present a peculiar and well-known appearance. 

 The outline is sharp, and dark, and well defined, but not nearly so wide 

 as that of the air bubble, because the difference of the refractive power 

 between the oil and the fluid, although very great, is much less than 

 that which exists between the air and the fluid medium which con- 

 tains it. Every one should compare carefully air bubbles with oil 

 globules under the microscope. Oil globules within cells, and free 

 oil globules of various sizes, as seen in milk, are represented in 

 pi. XIX, figs. 123 126. Every observer should be familiar with the 

 microscopical appearance of oil globules of different kinds. Cer- 

 tain kinds of fatty matter contain much crystalline fat, as stearine 

 or margarin, which is not a pure substance. These crystallise 



