MICROSCOPIC ANALYSIS. COLOUR. xxix 



and its sides are mostly simultaneously visible j but under a high power, only those parts 

 lying within a very limited vertical range, or in the same plane, are visible at one focus ; and 

 the parts lying in planes above or below this can only be brought into view by altering 

 the focus ; hence the views of objects under high powers con'espond to views of transverse 

 sections of the same objects made through various horizontal planes; and as the margins 

 of objects are usually more distinct by transmitted light than the upper surface, spherical or 

 rounded bodies frequently appear flattened. When several bodies of the same kind are 

 visible in the field of the microscope, some will almost always be found lying upon their 

 sides ; and even when the objects are greatly flattened, some will mostly be found lying on 

 edge, presenting the side view\ 



b. But as there may be uncertainty in regard to the relation of these bodies to each other, 

 the only safe method in forming a conclusion is to cause them to revolve or roll over, so that 

 all their aspects may be distinguished. This is in general easily accomplished ; if the object 

 be already immersed in liquid, the inclination of the stage will answer the purpose ; or a httle 

 aether, chloroform, naphtha, alcohol, or some other volatile liquid in which they are insoluble, 

 must be added. The currents produced by the evaporation of these will cause the objects, 

 especially such as are near the edges of the hquid, to move in all directions, and their true 

 form may be discerned. Sometimes moving the thin glass cover sidewise, the object being 

 kept in view, will answer the same end. 



c. In figures of microscopic objects, the side view should always be exhibited if possible ; 

 and if not, it should certainly be described. 



d. In the case of crystalline bodies, or such as present angular edges, their angles should 

 be measured with the goniometer, if then* chemical composition be unknown. 



2. The Colour. — The colour of objects should always be carefully described, and its cause 

 accurately determined. It most commonly arises from, — 1, partial absorption ; 2, the pre- 

 sence of pigment, or other colouring matter ; 3, from iridescence ; 4, from polarization, &c. 



1 . The most common cause is a peculiar property by which a portion of the coloured 

 rays composing the white light, which falls upon or is transmitted through an object, is 

 absorbed, the remainder being reflected or refracted so as to reach the eye. On examining 

 bodies thus coloured, with whatever powers, then' substance is found uniformly coloured, 

 and this colour is unchanged by their immersion in water or oil of turpentine, and is the 

 same in transparent bodies by both transmitted and reflected light. This is commonly 

 regarded as t\\Q iwoper colour of an object. Example : a crystal of blue vitriol. 



2. — a. In many cases, however, although an object may appear to the naked eye uniformly 

 coloured, on examining it with a high power, the colour, which in fact arises from the above 

 cause, is seen to be confined to certain molecules or granules, whilst the general substance 

 is colom-less. These granules may consist of vegetable or animal colouring matters, metallic 

 oxides, &c. The nature of these matters should always be determined, if possible, either by 

 microscopic chemistry — micro-chemical analysis, as it has been called, — or by ordinary che- 

 mical analysis. When the colouring matter is of organic nature, and when its com])osition 

 cannot be determined, or it has no definite name, it is called pigment. Objects coloured by 

 pigment, metallic oxides, or other colouring matters, are best examined by direct (not 

 obhque) transmitted hght, and when immersed in either water or oil of turpentine. These 

 liquids do not change the colour, nor destroy it unless the pigment be soluble in them ; but 

 by rendering the general substance of the object more transparent, they cause the granules 

 to become more distinct. The colom- is the same both by transmitted and reflected light. 

 Example : a brown or black hair of an animal. 



b. Sometimes bodies coloured by pigment or other colouring matters appear under the 

 microscope uniformly dyed, although the colouring matter consists of an insoluble molecular 



