Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/292

Rh 276 MICROSCOPE the inclination of the body ; but this is introduced into the port able form of the instrument shown in fig. 29, the basal portion of which (fig. 30) can be used, like that of the preceding model, as a eimple microscope, and, by a most ingenious construction, can be so folded as to lie flat in a shallow case (fig. 31) that holds also the upper part with the objectives of both the simple arm and the com- FIG. 28. Nachet s Combined Simple and Compound Microscope. pound body. M. Nachet now connects his objectives with the body of his microscopes, not by a screw, but by a cylindrical fitting held in place by the pressure of a spring-clip against a projecting shoulder. This method not only allows one objective to be re moved and replaced by another much more readily than does the screw-fitting, but also renders the centring of different objectives more exactly con formable. It may be safely affirmed that a very largo proportion of the microscopic work of the last half- century, which has given an entirely new aspect to bio logical science, has been done by in struments of this simple Continental type. A larger model, however, was from the first adopted by English opti cians ; and, as a typical example of the general plan of construction now most followed both in England ^ and in the United States, proved the im- Jackson- Zentmayer micro- scope of Messrs FlG - 29. Nacliet s Portable Compound Microscope. Ross _( fig. 32) may be appropriately selected. The tripod base of this instrument carries two pillars, between which is swung upon a horizontal axis (capable of being fixed in any position by a tighten ing screw) a solid &quot; limb,&quot; with which all the other parts of the instrument are connected, a plan of construction originally devised by Mr George Jackson. The binocular body, having at its lower end (as in fig. 24) an opening into which either of the Wenham prisms can be inserted, and at its top a rack movement for adjusting the eye pieces to the distance between the eyes of the observer, is attached to a racked slide, which is so acted on by the large double milled FIG. 30. Nachet s Portable Dissecting Microscope ; on the left as set up for use, on the right as having the stage P turned back upon the joint 0, so as to lie flat on the bottom of the case. head in the upper part of the limb as to give a &quot; quick &quot; upward or downward motion to the body; while the &quot; slow&quot; motion, or fine FIG. 31. Xachet s Portable Compound and Dissecting Microscope, as packed in case. adjustment, is given by means of the vertical micrometer screw at the back of the limb, which raises or lowers a second slide behind the rack. 1 The stage is supported upon a firm ring, which is immovably fixed, not to the limb, but to a strong conical pivot which passes through the limb, to be clamped by a screw-nut at its back, the purpose of this being to allow the whole stage to be inclined to one side or the other at any angle, so that a solid object may be viewed sideways or from below, as well as from above. Upon this ring the stage rotates horizontally, its angular move ment being mea sured by a graduated scale and vernier at its edge ; and it can be fixed in any azi muth by a clamping- screw beneath. Rect angular movement .; is given to the traversing platform which carries the ob- FIG. 32.- Loss s Jackson-Zentmayer Compound ject by two milled Microscope, heads on the right of the stage, the. whole construction of which is adapted to allow light of extreme obliquity to be thrown upon the object from beneath. On the strong pivot by which the stage is 1 In the older form of construction still retained by some makers the fine adjustment acts directly on the objective, the fi ting of which is made to slide up and down within the nose of the body : but this plan is attended with many disadvantages.