46 BEALE, ON THE MICROSCOPE. 
thing for themselves. The observation of nature accurately 
should be the great object to which all who possess micro- 
scopes and intelligence should aim. At the fifth lecture we 
get to work in earnest. The apparatus is all prepared— 
cements, preservative fluids, cells—and now the objects are to 
be examined. 
The following remarks on the importance of examining 
the same objects in various ways are sound, and the illustra- 
tions judicious : ~ 
“Many objects require examination in several distinct ways before an 
accurate idea of their general structure can be obtained. It is in many 
instances of the utmost importance to examine a body by reflected light as 
well as by transmitted light, and to observe the peculiarities of structure 
when it is surrounded with ar, or immersed in water, or in a highly refract- 
ing fluid, such as glycerine, oil, turpentine, or Canada balsam. Not less 
valuable is the information we derive from the application of certain chemical 
reagents, to the consideration of which I shall devote some time in a future 
lecture. ‘The microscopical observer must, however, feel that in order to 
make out the exact nature of any texture it is necessary to subject it to 
various different processes of observation, and to the action of certain chemi- 
cal reagents, according to the ¢ransparency or opacity, density, refractive power, 
and chemical composition of the specimen. So also he must submit the 
object to examination with high powers and low powers. 
“Appearance of the Same Olject examined in Air, Water, and Canada balsam, 
by Transmitted Light, and under the influence of Reflected Light and Polarized 
Light.—Under these five microscopes have been placed specimens of the 
same structure (spherical crystals of carbonate of lime) magnified in the 
same degree. 
“In Air,—In the first they are shown by transmitted light in air mounted 
in the dry way, and you will notice how very dark and thick their outline 
appears, and how impossible it is to make out the ultimate arrangement of 
the crystalline mass. 
“In Water.—In the second microscope the crystals are seen in water. 
The outline is still very dark and thick, but a few lines may be observed 
radiating from the centre of the crystals towards their circumference, 
although not very distinctly. 
“In Canada Balsam.—In the third microscope the crystals are shown im- 
mersed in Canada, balsam, Here the outline appears as a sharp well-defined 
line. A vast number of narrow lines are seen radiating from the centre of 
the crystals towards its circumference, which shows that it is really made 
up of a congeries of minute acicular crystals, 
“ By Reflected Light.—In the fourth microscope the crystals may be exa- 
mined by reflected light. Their globular form, and yellowish colour, are very 
distinctly seen, and you may observe that the surfaces of the crystals 
generally are slightly rough, while some appear to be covered by minute 
elevations. 
“ By Polarized Light.—Under the fifth microscope another preparation of 
the crystals is seen under the influence of polarized light. Each erystal 
exhibits a black cross which alters its position and appearance as the 
analyzer is rotated. These important points might be illustrated by a vast 
number of other substances, and I cannot advise you too strongly to subject 
various microscopical structures to exammination in air, water, and Canada 
balsam, and by direct or fais as well as under the influence of trazsmitted 
light, and in some cases by polarized light.” 
