204: 



THE AMERICAN MONTHLY 



[November, 



single lens of that focal length to 

 magnify lo diameters. Of course the 

 value of such an arrangement depends 

 on its authority. The table was made 

 for my personal convenience, and I 

 send it solely for the purpose of 

 gi^'ing a better idea of what I have in 

 mind. 



A tube of standard length (ten 

 inches) and a 2-inch eye-piece are to 

 he used in all cases where this table is 

 referred to. 



Liut'ur 

 Foca/ Length. Limits oj Magnifying Lititits of 



, ' ^ Variation. Power. Variation. 



4-in. 4.000 12.50 



3.7.50 13.39 



3* 3.500 14.28 



3.2.5U • • 15.47 



3 3.000 IG.eii 



2.750 18.33 



2A 2.500 20.00 



2.250 22.50 



2 2.000 25.00 



1.875 26.78 



Iv 1.7.50 28.57 



1.625 30.95 



lA 1.500 33.33 



1.375 36.66 



Ij 1.250 40.00 



1.125 4.5.00 



1 1.000 50.00 



.9375 53.57 



J .8750 57.14 



.8125 61.90 



5 .7500 66.60 



.7016 70.83 



S .6666 7.5.00 



.5833 87.50 



A .5000 100 00 



.4687 107.14 



i\ .4375 114.28 



.4187 119.64 



^5 .4000 125.0 



.3666 137.50 



?, .3333 150.0 



.2916 17.5.00 



J .2500 200.0 



.2250 22.5.00 



\ .2000 250.0 



.1833 275.00 



Jt .1666 300.0 



.1547 325.00 



i .1428 350.0 



.1339 375.00 



i .12.50 4(X).0 



.1180 425.00 



-O- 



VV.M. 



Kotary Object Carrier. 



BY J. M. FLINT, SURG, U. S. N. 



The following described device for 

 exhibiting a series of mounted mi- 

 croscopic objects, without the incon- 

 \enience of a change of slides, 

 though probably .not entirely new 

 (few things arc so), is yet original 

 so far as the writer is concerned, and 

 has been found efficient in practice. 

 As described it is arranged for show- 

 ing ff)raininifcra. wliicli are viewed 



as opaque objects, with a low power. 

 The selected foraminifera are 

 mounted on small brass disks fur- 

 nished with a stem, by means of 

 which they may be carried in a 

 ' Beck's disk holder ' when it is 

 desired to make a thorough study of 

 the specimens. 



Ordinarily these disks are inserted 

 in thin wooden slides of regulation 

 size and kept in the usual boxes made 

 for the purpose, until the series is 

 complete or ready for transfer. In 

 order to protect the specimens from 

 dust or injury, and at the same time 

 maintain their accessibility, movable 

 covers are constructed and secured as 

 follows : — A score or more of curtain 

 rings, not flattened, are slij^ped upon 

 a squared rod of wood, and brushed 

 over freely with thick shellac. On the 

 following day, before the shellac has 

 become hard, the rings are slightly 

 separated in pairs. When the pairs 

 are firmly united, a thin glass cover 

 is secured to the upper surface of 

 each pair, and thus a little box cover 

 is formed, deep enough to enclose 

 disk and specimen. Now, by driv- 

 ing two small gimp tacks into the 

 wooden slide, at the proper distance 

 apart, and deep enough so that the 

 heads of the tacks will just enter the 

 groove between the rings, a simple 

 catch is formed, by means of which 

 the cover may be secured, and also 

 be removable at pleasure. 



For exhibition — and for covenience 

 of reference as well — these disks, 

 bearing the specimens and the cov- 

 ers, are transferred to a thin circulai" 

 plate six inches in diameter, in this 

 case made of three or four sheets of 

 card-board glued one upon the other. 

 This makes a firm plate, not liable 

 to warp, and in which holes may be 

 readily bored for the insertion of the 

 disks, and the tacks driven to secure 

 the covers. By inserting the disks 

 as near the edge of the plate as pos- 

 sible, a line fifteen or more inches in 

 length is obtained on which to display 

 the objects. The circular plate bear- 

 ing the specimens as above, is made 



