MICROSCOPIC OBJECTS. 117 



of the former requiring almost central light, while the 

 latter requires it very obhque. From this cause artificial 

 illumination is to be preferred to daylight for this class 

 of objects, as the hght of a lamp or candle gives the rays 

 diverging without any apparatus whatever. The same 

 effect, however, may be produced in daylight, either with 

 Dr. Wollaston's or Dr. Goring's illuminator, where the 

 rays after meeting at the focus of theu' illuminating lens 

 are permitted to diverge, and by placing the object out of 

 the centre obhque vision is obtained. 



'•' In the investigation of the class of objects now to bs 

 described, direct parallel rays are preferable, and indeed 

 in most cases are essential ; and on this account they are 

 scarcely ever well defined by simple candle- or lamp-hght. 

 In these, therefore, a clear dayhght, directed through the 

 axis of the instrument, should be employed. 



"In the selection of the hair of animals for microscopic 

 examination, either as opaque or transparent objects, the 

 hghtest-coloured ones should be preferred, as they per- 

 mit us far more easily to observe their internal conforma- 

 tion, while the colouring-matter frete mucosumj in some 

 hair is so considerable as to render them incapable of 

 transmitting the most feeble ray, and these are therefore 

 unfit for this purpose unless mounted in Canada balsam. 

 Those on the inferior side of such animals as the mouse, 

 dog, &c., should be on this account selected. Like the 

 scales on insects, the hair from different parts of the 

 same individual varies considerably in structure. 



''(1 .) The hairs of the Common Mouse fMus domesticusj 



