'SO THE MICROSCOPE AND ITS REVELATIONS. 



thrown out of gear when a more rapid rotation of the stage by hand is 

 desired; and it bears a graduated circle at its margin for the measure- 

 ment of angles. It is fitted immediately beneath the object-platform 

 with an iris-diaphragm, worked by a lever action. 



75. Beck's Improved First-class Microscope. In order to meet the 

 demand for very oblique illumination, and to supply this in a mode yet 

 more perfect than the Zentmayer system, Messrs. Beck have adapted to 

 the preceding instrument a swinging sub-stage, carried by an arm that 

 works radially upon a large vertical disk attached to the limb, on the plan 

 originally suggested by Mr. Grubb; his semi-circle being extended, how- 

 ever, into a nearly complete circle, so as to allow the arm carrying the 

 sub-stage and mirror to be brought round to the upper Bide of the 

 stage, for the illumination of opaque objects. The essential feature of 

 their construction, however, which differentiates it from every other yet 

 devised, consists in a provision for adjusting the illuminating apparatus 

 to the thickness of the glass slide on which the object is mounted. This 

 is effected by making the disk with its radial arm, slide vertically in a dove- 

 tail fitting; the illuminating apparatus attached to it, at whatever degree 

 of obliquity it may be placed, being raised or lowered (by a lever-handle) 

 in the optical axis of the instrument, so as to enable the illuminating 

 cone to be exactly focussed in the object itself which on the Zentmayer 

 model, can only be done with precision when the upper surface of the 

 slide is exactly in the plane of the horizontal axis of the swinging ' tail- 

 piece.' The Stage also, in this elaborate instrument, is so attached to 

 the limb by a firm pivot, as to be capable not only of being inclined to- 

 ward either side at any angle, but also of being turned completely over, 

 so as to allow the object to be viewed from its under side a provision to 

 which the Author's experience makes him attach a special value. 



First-class Binocular Microscope-Stands, copied (more or less closely) from 

 either the Ross or the Jackson models, are also made by Messrs. Baker, Collins, 

 Crouch, Pillischer, and Swift, as well as by other makers of whose work the 

 Author has no personal knowledge. That of Mr. 

 provision for meeting the difficulty which is contii 

 the image in place during the rotation of the stage, 

 the adjustment which suits one Objective, not being good for another somewhat 

 differently centered. This defect presents- itself still more frequently when a 

 * nose-piece' is in use; its centering being rarely so exact as to be free from an 

 error that makes itself very perceptible when a high power is exchanged for a 

 low one. By means of two diagonal screws beneath the stage, worked by two 

 milled-heads at its hinder margin, Mr. Crouch affords a ready means by which 

 the observer can adapt the centering of his stage to any objective he may have 

 in use. Mr. Browning also constructs a First-class Stand for his Stephenson 

 Binocular. 



MICROSCOPES FOR SPECIAL PURPOSES. 



Of the large number of instruments which have been ingeniously de- 

 vised, each for some particular use, it would be quite foreign to the pur- 

 pose of this Treatise to attempt to give an account. A few forms, how- 

 ever, may be noticed, as distinguished either by their special adaptiveness 

 to very common wants, or by the ingenious manner in which the require- 

 ments of particular classes of investigators have been met. 



76. Dr. Beetle's Pocket Microscope. This instrument consists of an 

 ordinary Microscope-body, the Eye-piece of which is fitted with a draw- 

 tube that slides smoothly and easily; whilst its lower end is fitted into an 

 outer tube, of which the end projects beyond the objective. Against this 

 projecting end the object-slide is held by a spring, as shown in Fig. 53, 



