SCIENCE. 



495 



MICROSCOPY. 



Mr. C. Henry Kain thus describes in the August num- 

 ber of The American Journal of Microscopy, his new 

 mechanical finger for the microscope, which will be found 

 a useful addition to the instrument. 



A glance at the engraving will render the working of it 

 intelligible to all. It consists essentially of a slotted bar 

 which may be firmly clamped to the upper (immovable) bar 

 of the fine adjustment by means of a milled-headed screw. 

 Through the end of this is fastened a round rod, at such 

 a distance from the objective that, when lowered, the end 

 will not strike the stage. Over this rod slips a split tube, 

 to which is soldered at an angle, a smaller tube. Through 

 the small tube passes a rod carrying a glass hair at its~ex- 

 tremity. This rod is easily rotated by means of a milled 

 head. The capillary glass thread is attached to the ex- 

 tremity by means of beeswax. The arrangement of split 

 tubes was suggested by Mr. Edward Pennock, to take the 

 place of a binding screw which I had intended ; it is a 

 very neat and convenient affair, and much less clumsy 

 than the arrangement I originally proposed. It will be 

 noticed that the finger has no revolving collar, as it is quite 

 unnecessary, especially when the microscope is provided 

 with a revolving stage. By dispensing with the revolving 

 collar and making all movements depend entirely upon 

 the adjustments of the microscope, greater stability and 

 accuracy in working are secured. 



To use the finger, the point of the glass thread is first 

 brought into the focus of the objective, or nearly so, by 

 sliding the tube on the vertical rod and pushing or pulling 

 the rod carrying the glass thread until the desired position 

 is attained. It is not difficult to do this, and, having once 

 been done by hand, it does not have to be repeated, as all 

 further movements are made by the adjustments of the 

 microscope. Supposing now the point of the glass thread 



A NEW MECHANICAL FINGER. 



to be in focus ; by means of the fine adjustment throw 

 the focus ahead of the point, then, by means of the coarse 

 adjustment, rack down and search for the object you wish 

 to pick up. Having found the object desired, again bring 

 the point of the thread into focus by means of the fine 

 adjustment ; then rack down with the coarse adjustment 

 and pick it up. Now rack back with the coarse adjust- 

 ment, remove the slip on which the material is spread, and 

 place your prepared slip or cover upon the stage. Again, 

 by means of the fine adjustment, throw the focus ahead 

 of the object, rack down with the coarse adjustment and 

 search for the spot where you wish to deposit the object, 

 and, having found it, again focus the object, then rack 

 down with the coarse adjustment, and, when the object 

 touches the slide and has been placed in proper position, 

 fix it by means of a very gentle breath. There are many 

 other devices by which this useful little instrument may 

 be used for a variety of purposes, for a description of 



which we refer the reader to Professor Phin's journal. 



Pennock's Oblique Diaphragm.— The accompany- 

 ing engravings show a new form of oblique diaphragm 

 devised by Mr. E. Pennock, and described by him in The 

 American Journal of Microscopy (August, 1881). It is 

 designed to be attached to the under side of the stage for 

 shutting off all light except a small pencil from the mirror. 

 Its function is the same as Smith's > -shaped diaphragm. 

 It is an adaptation of Mr. Mayall's spiral diaphragm, 

 which was originally designed for use with condensers of 

 wide aperture, and was described in a recent number of 

 the Journal of the Royal Microscopical Society. 



It may be mounted in either of two forms : the one to 

 fit into the usual tube, which, in the cheaper microscopes, 

 is attached to the under side of the stage ; the other to 

 screw directly into the stage aperture. 



Fig. 1. 



A. Tube 1% inch in diameter, fitting into accessory 

 tube beneath stage. 



B. Upper plate (shown as under,) having radial slot. 



C. Under plate, having spinal slot. 



D. Screw joining the plates. 



The manner of using it tp obtain pencils of varying 

 degrees of obliquity will be sufficiently manifest from the 

 construction. 



FlG. 2 — PLAN OF I'PPER AND LOWER PLATES MOVING FULLY ON 

 EACH OTHER. 



CORRESPONDENCE. 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. No 7iotice is taken of anonymous communi- 

 cations.] 



To the Editor oj " Science." 



I do not like to see so great an authority as Faraday 

 misunderstood, as he evidently is by your correspondent 

 on page 459 of your journal, and that too, in a way 

 which he took particular care to caution against — as to 

 the law of gravitating action. That it acts inversely as 

 the square of the distance he fully believed and admit- 

 ted ; or, to use his own words, " I know it is so." 



If your correspondent finds difficulty to account by 

 this law for the return of the earth from aphelion to peri- 

 helion, let him try to account for the return of a stone to 

 the earth when thrown up into the air ; for precisely the 

 same explanation applies to both, the highest point of 

 the stone's path being "aphelion." The resistance of 

 the air need not be regarded, for, though it modifies the 

 stone's path, it does not affect the theory of the action of 

 gravity. Geo. B. Merriman. 



Rutgers College. 



