08 SECOND-CLASS MICROSCOPES. 



Animalcule -cage, when brought up to its lower edge. No action 

 surpasses that of this Magnetic Stage in the facility with which it 

 may be worked ; but the arrangement is liable to the serious 

 objection that when it is used with Sea- water or with Acid solu- 

 tions, the almost inevitable spilling of these occasions the forma- 

 tion of rust on the steel surfaces, which no gilding will prevent ; 

 whilst if the power of the magnet should become weakened, it 

 cannot be readily augmented. — Beneath the stage is a detached 

 Sub-stage for carrying a Diaphragm-plate, Achromatic Condenser, 

 Polarizing-prism, or other apparatus ; this sub-stage is capable of 

 being moved nearer-to or farther-from the stage, by means of a 

 milled-head working in a rack in the supporting bar ; and its fit- 

 tings can be precisely ' centered ' to the axis of the body by means 

 of adjusting screws. The Mirror is furnished with a double arm, 

 by the extension of which very oblique light may be obtained. — 

 Altogether this instrument may be recommended as combining 

 many of the advantages of the more complicated and more expen- 

 sive instruments with those of the simpler and less costly. 



47. Nachet's Student's Microscope. — Although for the reasons 

 already mentioned (p. 64 note) the Author has abstained from 

 noticing any Continental Microscope of the Third Class, yet he 

 feels it due to MM. Nachet to make special mention of the new 

 model of Student's Microscope which they have recently intro- 

 duced, as possessing excellencies which distinguish it from all con- 

 structions previously devised. The general build of this instrument 

 corresponds with that of the Student's Microscope of Messrs. Smith 

 and Beck (Fig. 35), except that it is upon a smaller scale, and is sup- 

 ported on a single pillar with a cradle-joint, instead of being swung 

 between two uprights. The Body is furnished with a draw- tube, by 

 which it is shortened for packing ; and instead of being itself attached 

 to the rack, its lower part is embraced by a tube which carries the 

 rack, so that this Single body may be readily drawn out and re- 

 placed by the Binocular already described (§ 29, Fig. 23). The 

 ' slow motion ' is given by an arrangement corresponding to that 

 already described in Smith and Beck's Microscope, which seems 

 to have been adapted from Continental models ; the milled-head 

 which acts upon it, however, is placed at the top of the sliding- 

 stem, so as to be near that which gives the rack-and-pinion adjust- 

 ment. The chief peculiarity of this instrument, however, lies in 

 its Stage, which the Author has no hesitation in pronouncing to be 

 the most perfect of its kind that has been yet devised. Its base 

 is formed of a thick plate, 3| inches square, having a large circular 

 aperture ; and on this is superposed a circular plate of 3 inches in 

 diameter, to which a rotatory movement, concentric with the optic 

 axis of the Microscope, can be given with great facility. In this 

 circular plate a disk of thin plate-glass is cemented with black 

 cement, the united thickness of the two around the central aper- 

 ture being not more than 1-Sth of an inch, so that light of the 



