SMITH AND BECK'S STUDENT'S MICROSCOPE. 103 



of this is graduated to inches and tenths. The advantages of this 

 arrangement will be explained hereafter (§ 43). The "fine" ad- 

 justment is effected by means of a milled head, situated just be- 

 hind the base of the stem that bears the limb ; this acts on a 

 screw, the turning of which (by a contrivance that need not be 

 described in detail) depresses the stem with the limb and body 

 attached to it, so as to bring the objective nearer to .the object ; 

 whilst if the pressure of the screw be withdrawn, by turning the 

 milled head in the opposite direction, the tubular stem (with the 

 limb and body) is carried upwards by a spiral spring in its in- 

 terior, thus increasing the distance of the objective from the ob- 

 ject. This adjustment is remarkable for its sensitiveness, and 

 for its freedom from any displacing action upon the image. The 

 only other peculiarity that need be noticed in this instrument, 

 is the mode in which the object is borne upon the stage ; for, in- 

 stead of resting against a ledge, it lies upon a kind of fork, 

 which slides in grooves ploughed out of the stage, and which 

 moves with such facility, that the pressure of a single finger upon 

 one of the upright pins at the back of the fork is sufficient to 

 push it in either direction. At the extremity 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 micro- 

 scope is placed in a horizontal position for drawing with the 

 camera lucida (§ 49), 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 in- 

 strument can easily be made to receive the addition of an achro- 

 matic condenser and of a polarizing apparatus ; it ma^/ also be 

 fitted with a traversing stage, but there is scarcely sufficient room 

 for its working, to render such an addition worth its cost.^ 



35. Smith and Beck's Dissecting Microscope. — A modification of 

 the preceding pattern has been made for the special purpose of 

 carrying on dissections under the Compound Microscope, without 

 any interference, however, with the use of the instrument for all 

 ordinary purposes. The general plan of the instrument (Fig. 22), 

 as will be at once apparent, is essentially the same as that of the 



' No working Physiologist or Naturalist can require, in the Author's opinion, a better 

 instrument than the above ; unless he be directing his attention to some particular class 

 of objects, which need the very highest microscopic refinements for their elucidation. 

 The cost of the instrument, fitted with two eye-pieces, condenser for opaque objects, 

 aquatic box, and stage-forcep's, is (with case) about £7 ; the cost of the objectives de- 

 pends upon their magnifying power and upon their angular aperture. Those most 

 serviceable for ordinary purposes are the IJ inch, f inch, -^*^ inch, and -J inch, whose 

 respective prices are jES, and 3, 5, and 6 guineas; the first and third, or the second 

 and fourth, of these may be selected in the first instance, and the others added at any 

 time ; the addition of the -J inch (which the unpractised microscopist is scarcely likely 

 to employ to advantage, and which is only useful for a very limited set of purposes) 

 may be postponed until it is really needed and can be effectually employed. Morecan 

 be seen of most objects by the proper management of such a ^ inch as iVIessrs. S. and 

 B. ijow supply, than could have been made out by the ^ of a few years back. These 

 opticians are now constructing a new pattern of Student's Microscope, complete ih 

 itself, with two good powers, which will be well adapted to the most important uses of 



