FIELD'S EDUCATIONAL MICEOSCCfPE. 97 



substituted two pillars (a form •which Messrs. Smith and Beck 

 still retain in their Large Compound Microscope, Fig. 29) for 

 the single pillar connected with the microscope itself by a " cra- 

 dle-joint" (as in Fig. 20) which was previously in use; but in 

 place of pillars screwed into the tripod base, a pair of flattened 

 uprights, cast in one piece with it, is now generally adopted, 

 with a view both to greater solidity and to facility of construc- 

 tion. Messrs. Powell and Lealand, it will be observed, adopt a 

 tripod support of a different kind (Fig. 28), still, however, carry- 

 ing out the same fundamental principle, of swinging the micro- 

 scope itself between two centres. Two different modes of giving 

 support and motion to the "body" will be found to prevail. 

 One consists in its attachment at its base to the transverse " arm," 

 which is borne on the summit of the movable stem, whose rack 

 is acted on by the pinion of the milled head, as in Figs. 18, 27, 

 28 ; whilst in the other, the body is supported along a great part 

 of its length by mieans of a solid " limb," to which is attached 

 the pinion that acts on a rack fixed to the body itself, as in Figs. 

 21, 22, and 29. The former method has the advantage of ena- 

 bling the body to be turned aside by the rotation of the trans- 

 verse arm upon the summit of the stem, — a movement which is 

 often convenient, both as leaving the stage clear for dissection, 

 &c., and as enabling the objectives to be more readily exchanged ; 

 but it is subject to the disadvantage, that unless the transverse 

 arm and the body are constructed with great solidity, the absence 

 of support along the length of the latter leaves it subject to vi- 

 bration, which may become unpleasantly apparent when high 

 powers are used, giving a dancing motion to the objects. With 

 a view of preventing this vibration, Messrs. Powell and Lealand 

 connect the top of the "body" with the back of the transverse 

 arm, by a pair of oblique " stays" (Fig. 28). The second method 

 of support is decidedly superior in steadiness, a perfect freedom 

 from tremor being obtained with less solidity, and therefore with 

 less cumbrousness ; the mode in which the rack is applied, more- 

 over, in the microscopes of Messrs. Smith and Beck (most of 

 which are constructed upon this plan) gives to it a peculiar 

 smoothness and easiness of working ; but the traversing move- 

 ment of the body is sacrificed. Although some attach considera- 

 ble importance to this movement, the author's experience of 

 instruments constructed upon both plans, leads him to give a 

 preference to the second. 



31. Field's Compound Microscope. — The first of the simpler 

 forms which we shall more particularly describe, is that to which 

 the medal of the Society of Arts has been recently awarded, not. 

 as a testimony to the perfection of its construction, but as mark- 

 ing the highest degree of excellence among the instruments sent 

 in competition, that seemed consistent with the cheapness^ which 



' The. price of this instrument, complete, with two eye-pieces and two achromatic 

 clijectives giving a range of power from about 25 to 200 diameters, condenser on a 



7 



