28 THE AMERICAN MONTHLY [February, 



similar performance is almost entirely nullified. In making the neces- 

 sary experiments some astonishing results appear. With a non-ad- 

 justable dry 1-5 corrected for a cover-glass of 0.16 mm., employing the 

 extremes of cover-glass which are used by the various manufacturers as 

 standard, as obtained by the efforts of Prof. Gage, I found that for 0.25 

 mm. a tube length of 6 inches is required to obtain the proper correc- 

 tion, while for a thickness of o.io mm. 13 inches of tube length is nec- 

 essary. In a 1-8 objective adjusted under the same conditions 4^^ inches 

 is the requisite for a cover of 0.25 mm., and for o.io mm. 15 inches. 

 The further fact is shown that with a 1-5, which, under conditions of 

 tube length and cover-glass given above, shows certain structure, well 

 defined, absolutely fails to show anything of it under a cover-glass of 

 O.IO mm. on one side, and 0.25 mm. on the other, and further a marked 

 chromatic ovei"- or under-correction. With a cover of 0.14, which 

 would seem but a slight variation from the standard, the objective is 

 spherically highly under-corrected, and with 0.18, highly over-corrected. 

 With objectives of high power the difference is still more marked. For 

 these experiments I have had Mr. J. D. Moeller, of Germany, mount 

 a series of Pleurosigma angulatu?n dry and Amphipleura pellucida 

 in balsam, under a series of covers varying from o.io mm. to 0.34 mm., 

 each carefully measured and marked. I have used these objects be- 

 cause they are my favorite tests, and it goes without argument in saying 

 that any preparation showing structure under above objectives will be 

 affected to the same extent by the varying conditions of cover-glass, as 

 these objects and in objects of still finer structure the limit of visibility 

 will be reached correspondingly sooner. 



The system which I have devised to aid in overcoming these difficul- 

 ties depends in the first instance upon a micrometer for measuring the 

 thickness of cover-glass. While the delicate instruments made by M. 

 Grossman, of Germany, are excellently sviited for this purpose, they are 

 expensive. I have endeavored to overcome this objection by construct- 

 ing a plain screw which, while not so sensitive to the touch, is sufficiently 

 so for all practical purposes. The instrument is provided with a stand 

 of japanned iron. Cut horizontally through the top is a thread of g^^r-ii^ch 

 pitch and y^g^-inch outside diameter. A recess is cut on the top below 

 the line of the screw and at right angles to it for placing the covers. 

 The.one-half of the top of the stand which receives the micrometer screw 

 is slotted longitudinally to the depth of the screw and is provided with 

 a set-screw to take up wear. The other half has the fixed screw, adjus- 

 table, however, for final adjustment. The end of the micrometer screw 

 is milled, but of a small diameter, so that no force can be exerted so as 

 to endanger the covei'-glass. Fixed on the screw between two nuts is 

 a brass drum with a ^-inch face. A knife-edge index finger is fixed to 

 the top of the stand and projects over the top of the drum. To the out- 

 side diameter of the drum is fixed a strip of glazed paper provided with 

 a series of divisions. The first gives the thickness of cover-glass in one- 

 thousandth inches, the second one-hundredth millimeter. The third 

 indicates the proper tube length with various thickness of cover-glass 

 with a non-adjustable \ corrected under a tube length of 8^ inches and 

 cover thickness of 0.16 mm. ; the fourth gives the tube lengths of a 1-5 

 inch objective under the same conditions ; the fifth for a ^, and the sixth 

 for a 1-12 for same conditions of tube length and cover ; the seventh is 

 for a 1-6 with the same cover and tube length of 160.0 nrm. 



