74 PEACTICAL TREATISE ON THE MICROSCOPE. 



ratus, whereby the quality of the light may be materially 

 modified. 



All the parts essential to a compound achromatic micro- 

 scope having now been described, attention wiU next be 

 directed to the different arrangements adopted by the prin- 

 cipal makers to render the mechanical part most effective, 

 and, as in all other cases, the names of the manufacturers 

 win, as far as practicable, be taken in alphabetical order, no 

 preference being given to the workmanship of one over that 

 of another, but credit always awarded wherever it may be 

 due. 



MESSRS. POWELL AND LEALAND'S ACHROMATIC COMPOUND 

 MICROSCOPE. 



This instrument, first described in the Microscopical Journal, 

 Vol. I., page 177, is represented by fig. 43 ; it stands on a 

 firm tripod base of brass, on which is a circular plate ; to this 

 two stout pillars are attached, bearing at their upper extremi- 

 ties the ends of the trunnions, upon which a strong piece of 

 metal, giving attachment to the compound body and the 

 stage, is supported; by means of the circular plate, the pillars 

 can be turned upon the tripod, and the weight of the com- 

 pound body and stage brought over one or more of the feet of 

 the tripod, and the instrument, therefore, rendered more 

 steady. This plan of using the double pillar was first adopted 

 by Mr. George Jackson, in 1838, and possesses the advantage 

 of being light and of distributing the weight of the superin- 

 cumbent parts more equally on the tripod than where only one 

 piUar is employed. The compound body is supported nearly 

 the whole of its length on a strong arm, having a hoUow frame 

 at its top, after a plan first described by Mr. Powell, in the 

 fifty-third volmne of the Transactions of the Society of Arts. 

 The coarse adjustment is made by a rack and pinion contained 

 within the frame above noticed; and the latter turned by 

 the large milled head A. In order that the compound body 

 may be moved easily and still be very steady, it is attached 

 to a cradle resting upon two rollers, one inch-and-a-quarter 



