130 SELECTION AND USB 



The changes which are produced in some objects when the 

 light is made to full on then in different directions are very 

 marked. Thus, for example, the mineral known as specular 

 iron ore, when illuminated by light falling on it in one direc- 

 tion, is brilliant in the extreme, while when the light falls in 

 other directions it is dead and lustreless. Arid as it is not 

 always convenient to change the position of the lamp, it is a 

 great advantage to be able to tarn the object round. The sim- 

 ple contrivance just described enables us to do this perfectly. 



A more perfect arrangement, intended for the same purpose, 

 has been devised by Mr. Beck, of London. Mr. Beck's is, 

 however, more expensive than ours. 



Plain Slides. The common plain slides serve very well 

 for examining ordinary deposits in liquids. This is particu- 

 larly the case where inanimate objects, vegetables and minerals 

 are to be examined. Active animals require some contrivance 

 for keeping them still. 



Tlie Concave Slide, as it is called, is simply a thick 

 slide with a cup-like hoilow ground in the centre. Such slides 

 are cheap, and very convenient. A drop of water placed in one 

 of these concaves, and covered with a thin glass, may be exam- 

 ined easily and thoroughly with moderate power. It is 

 sometimes desirable to employ a cell with a perfectly flat bot- 

 tom of very thin glass. Such cells may be easily and conve- 

 niently made out of a slide of metal, or preferably of vulcanite, 

 through which a hole the size of the proposed cell has been 

 pierced. A piece of thin glass may then be cemented to the 

 under side of the slide, so as to form a water-tight cup. The 

 hole in our slides is round, and has, on the under side, a seat 

 or rebate, a little larger than the hole itself. In this rebate a 

 round glass cover fits, so as to leave the under side of the slide 

 perfectly smooth. Such cells are very convenient, as they are 

 easily cleaned, and are not difficult to repair when the thin 

 glass gets broken. The liquid is also easily covered by means 

 of a thin glass cover, and when full, considerable inclination 

 may be given to the slide before the liquid shows a tendency to 

 run out. Various other devices of a simple kind may be con- 

 trived by the microscopist for similar purposes. 



