DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 89 



being attended to, and the triangular bar, b, sufficiently 

 raised, we may proceed with perfect safety. On the stage 

 is fitted, in a similar manner, by means of two smaller 

 pins, a fork-shaped plate of brass, shewn apart at fig. 13. 

 This plate is designed to retain the slider firmly on the 

 stage when the instrument is inclined. The focus is ad- 

 justed by means of a rack and pinion; the letter, zy 

 denoting the large milled head of the latter. The 

 mirror-frame slides up and down the stem, c, thus allow- 

 ing the angle of the pencil of light to be increased or 

 diminished. 



The stand of this instrument appears not to be different 

 from those of the old-fashioned compound microscope, ex- 

 cept in having a simple round foot, which is here adopted 

 to save room and expense. Like them it has a joint for in- 

 clining the instrument at any required angle ; but it has, 

 besides, this novelty, which, although simple, is of great 

 importance, viz. a capability of permitting the stem, c, 

 to be turned round on its axis, so as to allow a glass 

 vessel, or slider accommodated for animated creatures 

 or plants, to be retained in an upright position, and 

 their contents investigated without having recourse to 

 stoppers or corks. When any of these vessels are used 

 in ordinarily -constructed microscopes, they require to 

 be placed on their sides, which, independent of disturb- 

 ing the objects, admits not unfrequently into the field of 

 view, small portions of the air included in the vessels, so 

 as greatly to interrupt our view. When observations of 

 this nature are to be made, the stem, c, should be turned 



