78 FIRST-CLASS MICROSCOPES. 



the axis of the body. — The special advantages of this instrument 

 consist in its perfect steadiness, in the admirable finish of its work- 

 manship, and in the variety of movements which may be given both 

 to the Object and to the fittings of the Secondary Stage. Its disad- 

 vantages consist in the want of portability that necessarily arises 

 from the substantial mode of its construction ; and in the multi- 

 plicity of its movable parts, which presents to the beginner an 

 aspect of great complexity. This complexity, however, is much 

 more apparent than real ; for each of these parts has an indepen- 

 dent action of its own, the nature of which is very soon learned ; 

 and the various milled-heads are so disposed that the hand readily 

 (and at last almost instinctively) finds its way from one to the other, 

 so as to make any required adjustment whilst the eye is steadily 

 directed to the object. To the practised observer, therefore, this 

 multiplication of adjustments is a real saving of time and labour, 

 enabling him to do perfectly and readily what might otherwise 

 require much trouble, besides affording him certain capabilities 

 which he would not otherwise possess at all. 



54. Powell and Zealand's First-class Microscope. — The earlier 

 form of this instrument, represented in Plate v., is far lighter 

 than the preceding in its general ' build,' without being at 

 all deficient in steadiness ; it has not, however, some of those 

 movements for which Mr. Ross's plan of construction is especially 

 adapted. The tripod stand gives a firm support to the trunnions 

 that carry the tube to which the stage is attached, and from which 

 a triangular Stem is raised by the rack-and-pinion movement set in 

 action by the double milled-head, whereby the 'coarse' adjustment 

 of the focus is obtained.* The triangular stem carries at its sum- 

 mit the transverse Arm, which contains (as in Mr. Ross's Micro- 

 scope) the lever-action of the ' fine ' adjustment ; and this is acted 

 on by the milled-head at the back of the arm, whence also pass two 

 oblique stays, which, being attached to the upper part of the Body, 

 assist in preventing its vibration. The Stage is provided with a 

 traversing movement in each direction, to the extent of about 

 three-quarters of an inch ; this is effected on the plan known as 

 Turrell's, in which the two milled-heads are placed on the same 

 axis, instead of side by side, one of them being also repeated on 

 the left hand of the stage, so that the movements may be commu- 

 nicated either by the right hand alone or by both hands in com- 

 bination. The Platform which carries the object is made to slide, 

 as in the preceding case, on the summit of the traversing appa- 

 ratus ; and it has not only a ledge on which the object may rest, but 

 also a ' spring-clip ' for securing the object whenever the stage may 

 be placed in a vertical position. This platform, moreover, is so con- 

 nected with the traversing apparatus, that it may be turned round 

 in the direction of its plane : but as this rotation takes place above 



* A smaller and lighter form of this instrument is made, in which 

 the legs fold together, so that it admits of being packed into a flat case. 



