490 APPENDIX. 



microscope ; on the other hand, when it is desirable to have 

 the hght near the base of the instrument, the stem, c d, 

 unscrews, and the gallery, f, screws into the socket, b; 

 a shallow argand burner, e, screws into the gallery, /; this 

 gallery carries an inner rim, into which fits a Leblond's 

 patent achromatic blue lamp glass, h ; a piece of coarse wire 

 gauze, represented by the arched dotted lines, g, fills up the 

 central aperture, so as to prevent irregular currents of air 

 affecting the steadiness of the flame; the outer rim of the 

 gallery supports a copper cylinder, i, 8 inches high, and 3^ 

 inches in diameter ; in this, and opposite the burner, a circular 

 aperture, 2 inches in diameter, is cut; outside this cyhnder is a 

 rim, k, on which an outer cylinder, I, rests and rotates round 

 the inner one, i ; this has a corresponding aperture, into which 

 is fitted a disc of neutral tint glass, m ; the inner blue glass, h, 

 absorbs the yeUow rays of the flame to a great extent, but it 

 still gives a glaring objectionable light, which the glass, m, cor- 

 rects, and the combination of the two tints affords a soft white 

 light, that renders the definition very distinct. When it is 

 desirable to diminish the beam of light, it is effected by 

 rotating the cylinders, and as the two circles cut each other 

 the aperture may be contracted to any extent; to facilitate 

 the rotation, an ivory knob, n, is attached to the outer cylin- 

 der, I, and another, o, to the inner one, i. 



This lamp would likewise be of service when making 

 minute dissections; also, for fine mechanical work, as in 

 watchmaking, &c. The lamp may be obtained of Messrs. 

 Smith and Beck. 



M. sachet's microscope foe chemical observations. 



This very valuable instrument is represented in fig. 265, and 

 is composed of a sohd foot, X, in which is fixed the piece, O, 

 for the reception of the small plate, P, on which the objects to 

 be examined are placed ; as in this instrument we view the 

 under surface of objects, the illumination must be on their 

 upper surface ; for this purpose we fix at pleasure to the piece 

 O the rod T, which holds the mirror, M, and the piece, D, for 



