ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 151 



of a special arrangement. In the middle of the bridge is screwed a short 

 steel cylinder, which extends into the central hollow of the spring for 

 about 8 mm. It possesses a cylindrical cavity, the longitudinal axis of 

 which coincides with that of the cylinder, and which ends conically. 

 Within the cavity is placed a small steel rod, the diameter of which is 

 somewhat smaller, so that the space is not quite filled. This rests with its 

 lower conical extremity in the conical end just alluded to. To the upper 

 squared off end of the rod, which is flush with the upper surface of the 

 bridge, is fixed a small cap in which the point of the micrometer-screw is 

 adjusted. The cap (fig, 17) is intended to protect the parts inclosed from 

 dust, and also to give a firmer hold to the finger. 



The following is the action of this contrivance. The cylinder which 

 extends from beneath the bridge to the central cavity gives a firm hold to 

 the spring. The rod in the steel cylinder forms the direct continuation 

 of the micrometer-screw with the advantage of a movable point, and as the 

 rod is of less diameter than the cavity in which it lies, the screw at its 

 lower end is practically movable on all sides. The arrangement offers the 

 following advantage : — If the point of a micrometer-screw is not quite 

 accurately placed, either in consequence of imperfect work, or from warping 

 in hardening, the screw will exert not only a vertical but also a lateral 

 pressure. This will in turn produce a lateral displacement of the tube. 

 If, however, the screw works on an easily movable point which is firmly 

 united to it, and has only a slight lateral movement, vertical motion only 

 will be communicated. Another advantage is, that the friction of the 

 micrometer-screw is as small as possible, and therefore a very strong spring 

 can be inserted without the scrow losing its easy regular action. In 

 consequence also of the power of the spring, the prisms a and b can be 

 fitted so close as to create a relatively strong degree of friction. 



(2) Eye-pieces and Objectives. 



Finding the general character of the Components of a Cemented Com- 

 bination Lens.* — Mr. E. M. Nelson premising that it is very useful to know 

 whether a combination consists of two or three lenses, and if those are 

 biconvex, plano-convex, meniscus, &c., gives directions for obtaining such 

 information without uncementing. The method employed is simply the 

 consideration of the reflected images from the surfaces of the glass. 



" Take the plane mirror of your Microscope in your hand, and examine 

 the reflection of a window. Notice that it is an erect image, and that wheu 

 you move the mirror in a certain way the image appears to come towards 

 you. Now look at the concave side, the image is inverted, and when the 

 mirror is moved in the same direction as before the image goes away from 

 you. A convex mirror behaves as a plane mirror, there being only this 

 difference — that the greater the convexity the smaller is the image, which 

 difference is also true of a concave mirror — viz. the greater the concavity 

 the smaller the image. If you now examine a single biconvex lens, you 

 will see a large erect image from the surface next the window, and a small 

 inverted image from the surface on the other side. It acts precisely as 

 if it were a convex and a concave mirror. In a single biconcave lens you 

 have a large inverted and a small erect image. In a plano-convex, with 

 the convex side towards the window, you will find a small erect image 

 from the convex side, and a large inverted image from the plane side. 

 With the plane side towards the window, you will have a large erect image 

 from the plane s'de, and a small inverted one from the other side. With 



* Engl. Meeli., xliv. (1886) pp. 320-1 (3 figs.)- Jouru. Quek. Micr. Club. iii. (1887) 

 pp. 13-7. 



