(16 SECOND-CLASS MICROSCOPES. 



of the fork is sufficient to push it in either direction. At the exti'e- 

 mity of one of the prongs of this fork is a ' spring-clip ' for securing 

 the object by a gentle pressure, which is particularly useful when the 

 Microscope is placed in a horizontal position for drawing with the 

 Camera Lucida (§ 71), the stage being then vertical. And at the 

 extremity of the other prong is a hole for the insertion of the pin 

 of the Stage-Forceps, which thus gains the advantage of the sliding 

 movement of the fork, in addition to its own actions. — This instru- 

 ment can easily receive the addition of an Achromatic Condenser, 

 Paraboloid, and Polarizing apparatus ; it may also be fitted with a 

 Mechanical Stage, but the range of its movement is limited. 



46. Ladd's Student's Microscope. — The ingeniously-constructed 

 Microscope of Mr. Ladd (Fig. 36) deserves special mention, on 

 account of the peculiarities which distinguish it from all other 

 forms previously devised. In the first place, it is remarkable for 

 its lightness ; the tripod Foot being constructed of a framework of 

 tubes securely braced together, and the other parts of the instru- 

 ment being made with less than their usual massiveness, yet with- 

 out such weakness as would permit vibration. The Body is attached 

 for a considerable part of its length to a frame which slides up and 

 down on a dovetailed bar ; and motion is given to this, not by a 

 rack -and -pinion, but by a chain working round a spindle turned 

 by the milled-head. This arrangement gives a movement of re- 

 markable smoothness ; and it has this great advantage over the 

 ordinary plan, that its action cannot become loose by wear, as that 

 of the best rack is liable to do in the lapse of years at the part 

 in which it has been most used ; the whole chain being tightened, 

 if necessary, by a small screw at the top, so that there need never 

 be the least amount of 'lost time.' Around the neck of the right- 

 hand milled-head there is a collar to which is attached a short 

 Lever : this collar is ordinarily so loose that the lever hangs verti- 

 cally as in the figure, and is not altered in position by the rotation 

 of the milled-head ; but by turning the nut at the lower end of 

 the level', the collar is made to press tightly round the neck of the 

 milled-head, and the lever may then be used to give a ' slow 

 motion ' to the body, by which its focal adjustment may be 

 effected with a nicety quite sufficient for all but the very highest 

 powers. In fact, for the ordinary purposes of Scientific investiga- 

 tion not requiring the use of those powers, the Author is disposed 

 (from much experience of this instrument) to prefer the slow 

 motion given by a lever to that given by a screw ; as it enables 

 the observer, when looking at an object whose different parts are at 

 different distances, to pass more readily from one focus to another, 

 and so more intimately to connect the different views together in 

 his mind. In using this lever, it is advantageous to give the hand 

 a bearing upon the tripod stand. The Stage is furnished with the 

 Magnetic bearing first brought into practical use by Mr. Gr. Busk. 

 This consists of a horse-shoe magnet screwed to the under-side of 



