THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. hi. 



Kew York, April, 1882. 



No. 4. 



Home-made Apparatus for Col- 

 lecting. 



As the collecting season once more 

 draws near, the description of a very 

 convenient bottle-holder which I de- 

 vised and used last season, may not 

 come amiss. Select an ordinary gim- 

 let-pointed wood-screw of the proper 

 size, flatten the head — or have the 

 blacksmith do it for you — then cut a 

 narrow strip of sheet-brass, bend it 

 round the bottle you intend to use, 

 turn the ends at right-angles to the ring 

 and let them project about ^ of an 

 inch, place the flat head of the screw 

 between them, and fasten all together 

 with a copper-rivet. You then have 

 an instrument somewhat resembling a 

 pocket cork-screw that can be readily 

 attached to an ordinary cane, or any 

 convenient stick (fig. 22), first winding 



Fig. 22. 



the stick for a short distance with 

 strong tv/ine to prevent splitting. A 

 ring for a net, hook-knife, or a spoon 

 can also be made in a similar way. 



I also find an ordinary gimlet- 

 pointed closet-hook screwed into a 

 stick, very handy for securing plants 

 floating in the water. 



E. L. Cheeseman. 



KOWLESVILLE, N. Y. 



Telescopic Field and Micro- 

 scopic Aperture. 



BY HON. J. D. COX, F. R. M. S. 



The discussion of the question of 

 angular aperture has resulted in a 

 general agreement among scientific 

 men that apertures in excess of the 

 maximum for dry glasses, are both 

 practicable and useful; and there is 

 a strong tendency to accept Profes- 

 sor Abbe's numerical aperture as a 

 convenient standard by which to ex- 

 press the comparative apertures of all 

 objectives, because it reduces them to 

 a common measure of the number of 

 rays which may pass from the illumi- 

 nated object in front of the lens, to 

 the image at the aplanatic focus be- 

 hind it. 



Accepting these results, there still 

 remains the question: What is the cor- 

 rect method of measuring this aper- 

 ture ? From a former generation of 

 microscopists we have inherited the 

 habit of measuring the angular aper- 

 ture of the microscope by an angle of 

 which the apex is the centre of the 

 microscopic field of view, and whose 

 sides bound the telescopic field of 

 view when the microscope is turned 

 into a telescope, either by removing 

 the ocular and looking down the tube 

 with the naked eye, or by substitut- 

 ing a terrestrial eye-piece in its place. 



The old sector table is the basis of 

 measurement for all the apertometers 

 in use, and the principal modification 

 since made, consists in the addition 

 of a semicircular disc of crown-glass, 

 or a hemisperical traverse lens, by 

 which the angle of aperture in glass, 

 or in a homogeneous-immersion me- 

 dium, may be directly measured. It 



