CONSTRUCTION OF THE MICROSCOPE. 63 



object-slide an arrangement than which, in the Author's opinion, 

 nothing can be better. This may be replaced by a cylindrical fitting for 

 the reception of a Polariscope, Paraboloid, etc. The double mirror is 

 carried upon an arm which swings on a pivot from the front of the limb 

 beneath the stage, and is capable of extension by a dovetail sliding bar. 

 Altogether, this instrument (so far as its mechanical arrangements 

 are concerned), comes nearer than any others that the Author has seen, 

 to his idea of a model Student's Microscope. 1 



61. JVachefs Student's Microscope. This instrument deserves special 

 mention for certain peculiarities of construction which distinguish it 

 from the ordinary Continental model of Microscopes of this class. 

 While most of these can be used only in the vertical position, the Micro- 

 scope of MM. Nachet is attached to the supporting pillar by a cradle- 

 joint, which allows it to be inclined at any angle. The body is furnished 

 with a draw-tube, by which it is shortened for packing; and is embraced 

 by a tube which carries the rack, so that it is well supported, and maybe 

 readily drawn out and replaced by the Binocular already described. 

 ( 38, Fig. 28). The 'slow motion' is given by a milled-head placed at 

 the top of the sliding-stem, so as to be near that which gives the rack- 

 and-pinion adjustment. The chief peculiarity of this instrument, how- 

 ever, 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. 2 Its base is 

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

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

 diameter, to which a rotary 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 aperture being not more than 

 l-8th of an inch, so that light of the greatest obliquity can be trans- 

 mitted to the object from beneath. The rotating plate is furnished with 

 a projection at the back, to which is attached a strong V-shaped pair 

 of springs, having their extremities armed beneath with small ivory 

 knobs, which press down on the Object-carrier. This last consists of a 

 brass frame furnished with tongues and springs projecting forward for 

 the reception of the slide, and also with a pair of knobs, to which the 

 fingers may be applied in giving motion to it; whilst the frame incloses 

 a piece of plate-glass a little thicker than itself. Thus the under surface 

 of the glass-plate of the Object-carrier slides over the upper surface of 

 the circular glass stage-plate; being held down upon it and retained in 

 any position by the pressure of the ivory knobs. The advantages of this 

 arrangement lie (1) in the perfect facility with which the Object-carrier 

 may be moved, and the steadiness with which it keeps its place when not 

 unduly weighted; (2) in the facility with which it can be readjusted, in 

 case the movement should become too easy, by bending down the V 

 springs; and (3) by the absence of liability to derangement by rust a 

 point of great importance when work is being done with sea- water or 

 chemicals. The front portion of the rotating plate bears a small pro- 



1 This Microscope, with two Eye-pieces, and with fairly good Objectives of 

 2-3ds and l-5th inch, is sold in New York for 35 dollars, or little more than 7. 

 It could probably be made in this country (if there were a considerable demand 

 for it) for 5 guineas. 



2 This Stage, which, on the Author's recommendation, has been copied, first 

 by Mr. Crouch, and now by other English opticians, seems to have been originally 

 invented by Mr. Zentmayer of Philadelphia. 



