232 THE AMERICAN MONTHLY [October, 



and ordinary objectives. I have so arranged it that the compensating 

 eve-lenses of diHerent foci are interchangeable. In fact, no special lens 

 is required ; you use your ordinary working eye-piece, whatever that one 

 may be. This is, of course, a great advantage ; bacteria, for instance,, 

 require a high power eye-piece micrometer, while such a power would 

 be useless on an ordinary object. Therefore, the ability to regulate 

 your eye-piece power to the object to be measured, will meet a long 

 felt want. 



Next let me say that I entirelv disapprove of having two movable 

 threads; at the outset ''the constant of the instrument " would be 

 doubled ; moreover, I am confident that a movable zero is a mistake. 

 I have, therefore, considerably altered this portion of the instrument 

 bv making the screw portion, together with the fixed zero thread, 

 movable in the other part, which might be aptly termed " an eye-piece 

 adapter." By this we secure the advantage of the double movable 

 thread, without the additional error of the double movable thread, and 

 this, moreover, without losing the convenience of a fixed zero. This 

 enables you to span your object at equal distances on either side of the 

 optic axis without disturbing the centricity of the eye-piece. A guid- 

 ing line has been added, because an error might creep in unless meas- 

 urements are made with precisely the same portion of the wires. 



The divisions of the screw-head have been made white on a black 

 ground, on account of their being easier to read in a darkened room. 

 A cap to protect the thread from dust and injury, etc., is provided as 

 the threads are no longer inclosed between the lenses as in the Huy- 

 ghenian form. An iris diaphragm is placed below the threads and as 

 close to them as possible. 



In spanning the stage micrometer it will be found better to take the 

 readings from centre to centre of the lines, by doing which you avoid 

 the dirtraction which is always present at the edges. 



The measurement of all objects should be performed under a wide- 

 angled cone of illumination so that the diflraction at the edges may be 

 minimized as much as possible. 



NOTES. 



Materials of the Microbe-Raiser. — Some of the means and meth- 

 ods of the micrologist, in his researches, must be mentioned. His outfit 

 is extensive and novel. It includes the best known microscopes and a 

 well-constructed incubator with heater and thermometer, numerous 

 test-glasses, beakers, filters, acids, alkalies, deep-colored dyes, and a 

 good supply of prepared cotton. 



In studying the life-history of his microbes he will require a well- 

 supplied commissariat. He must be a professional caterer and a boun- 

 tiful feeder. He must have fluids, semi-fluids, and solids, broths of 

 various meats, peptonized food, the serum of blood, a la Koch., and 

 Pasteur's favorite recipe with the French refinement : Recipe, lOO parts 

 distilled water, lo parts pure cane sugar, i part tartrate of ammonium, 

 and tiie ask of i part of yeast. Among the substantial must be found 

 boiled white of egg, starch, gelatin, Japan isinglass, and potato — the 

 last, from South as well as North America. — From Invisible Assail- 

 ants of Health., by Samuel Hart., M. Z>., iti The Popular Science 

 tJilv for October. 



