June 14, 1900] 



NA TURE 



155 



Another coarse adjustment improvement has been made 

 by Herr Reichert. Its principle is a rack and pinion 

 of specially hardened gun-metal, and a very important 

 feature is the springing with adjusting screws for tighten- 

 ing up. Of these there are three : one being used for 

 regulating the grip on the tube-mount, the others for 

 tightening the pinion ; thus the unavoidable wear and 

 tear can be compensated for. 



Fine adjustments, owing to the rigorous, requirements 

 of high-power photomicrography, have received very 

 great attention. ^ Modern advances may, however, be 

 reduced to some five main types: (i) the direct-acting 



screw, (2) the same with lever interposed, (3) the differ- 

 ential screw, (4) Reichert's lever fine acting, ('5) Berger's 

 endless screw. The first of these is seen in the Zeiss' 

 microscope (1886), where a left-handed micrometer 

 screw with a hardened steel point presses on a hardened 

 steel plate. In this, one revolution of the milled head 

 causes a movement of i /loi inch. The second is met with 

 in Messrs. Watson's microscope, where a lever of the first 

 kind with unequal arms is interposed. This arrange- 

 ment not only greatly reduces the weight bearing on the 

 fine adjustment, but slows the speed down to 1/350 inch 

 for one revolution. This speed is not at all too slow, as 

 any lens possessing a fair optical index is excessively 



nzL 



sensitive to focal adjustment when a | illuminating cone 

 is used. The differential screw fine adjustment occurs 

 in the microscopes of Messrs. Baker and Messrs. Swift. 

 Its advantage is that fine movements are obtained by the 

 use of coarsely threaded screws, thus reducing the 

 liability to wear and tear. Any degree of slowness may 

 be obtained, but 1/200 inch is adopted by the makers. 

 Herr Reichert's fine adjustment is especially suited to 

 the Continental model, and consists of an mgenious 

 adaptation of a double lever of the second order to the 

 usual direct-acting screw (Fig. 2). It is arranged for 

 movement of 1/200 inch. 



For fuller information see " On the Evolution of the Fine Adjustment," 

 . 3 Mr. E. M. Nelson (Journal R.M.S. 1899, pp. 366-373). 



NO I59S, VOL. 62] 



Herr Berger's (Zeiss) is another great improvement ii> 

 fine adjustments, and is on an entirely novel plan. The 

 extremity of the micrometer screw comprises a hori- 

 zontal toothed-wheel, which is actuated by an endless 

 screw, terminating in the usual micrometer knobs. 

 This arrangement permits of any degree of sensitive- 

 ness, and is ingeniously packed away in a hollow part 

 of the limb, so that the fine adjustment is both dust- 

 proof and out of risk from any accidental injury. A 

 valuable feature in the contrivance is a device for pre- 

 venting strain on the fine adjustment from over-winding. 

 This particular microscope is intended for the highest 

 class of work, and possesses another good feature, in 

 that the arm can be made to extend to any length over 

 the stage without increasing the pressure on the fine 

 adjustment. Thus an error in the original design of the 

 Continental model, viz. the shortness of the distance 

 between the limb and the optic axis, is corrected. 



In Pillischer's international microscope a serious 

 attempt has been made to reduce the inconveniences 

 inseparable from stage-clips. He secures these to a bar, 

 which, by means of a flange piece, raises or lowers then> 

 simultaneously^ so that their points press on the object 

 slide with any desired degree of pressure. 



The use of large slides has led to a corresponding 

 increase in the size of the stage, as well as in improved 

 stage-finders and mechanical movements. There is a 

 marked tendency to make the stages completely rotatory. 

 Among special stages, that of Herr Kraus (Reichert 

 and Co.) is perhaps the most notable. It is heated by an 

 electric current passed through a coil of platinum im- 

 mersed in the liquid paraffin with which the stage box 

 is filled. Regulation is accomplished by an ingenious 

 contact thermometer, a device which resembles an 

 ordinary thermometer, except that it is open at the toj> 

 end. A platinum thread can be set in the tube at any 

 desired temperature, and, when the mercury has risen 

 and has met this thread, an automatic arrangement 

 switches off the supply current. If the platinum and 

 the mercury break contact, the current re-enters. The 

 apparatus is said to be capable of rapidly producing and 

 sustaining a temperature constant to o'i° C. 



In the department of lenses, Messrs. Watson have 

 brought out their holoscopic eye-pieces, in which one 

 lens-mount slides within the other, telescope fashion, 

 thus forming a very convenient arrangement, easily 

 adjustable as an over-corrected or under-corrected ocular 

 as desired. The appropriate graduations are engraved 

 on the tube. 



Messrs. Zeiss have issued, at Dr. Hartwig's suggestion, 

 a very useful series of low-power objectives, callea 

 " planktonsearchers," which are to be immersed in a 

 trough of water, and so used for exploring it. They are 

 made of Jena glass, and present an image completely 

 plane and free from astigmatism quite close up to the 

 periphery. The microplanar series of objectives by the 

 same firm are used for projecting micro-slides on a 

 screen, and give a wonderfully good ten-feet picture 

 remarkably plane, sharp and well-defined, even to the 

 limits of the field. 



Another important set of objectives is Messrs. Leitz' 

 series of achromatics. These are made of Jena glass 

 free from fluorite. They are so well designed and cor- 

 rected that they give results but very little inferior to the 

 best apochromatics. Their freedom from fluorite renders 

 the risk of atmospheric deterioration insignificant. More- 

 over, they are sold at so low a price that Messrs. Leitz 

 must be admitted to have outstripped all other opticians 

 in this particular detail. 



Mr. H. J. Grayson, of Melbourne, has by an unknown, 

 process produced some very fine examples of ruled test- 

 plates, the rulings being executed in various gradations- 

 of fineness, as far as 60,000 lines per inch, and 2000 lines, 

 per millimetre. The accuracy of the work is remarkable 



