96 



THE AMERICAN MONTHLY 



[April. 



electric light is now generally used for illumination, and the instrument 

 is therefore known as the photo-electric microscope " (p. 354) 



" The relation between the Microscope and Telescope ma}^ 

 be impressively illustrated by the pupil with the following simple 

 apparatus : 



" In the figure, S represents a screen of card-board, through which 

 a cross, with arms about half an inch long, has been cut. This cross 

 is illuminated by a gas-jet L. O is a lens (a large pocket lens will 

 answer), which is placed eight or ten feet from S, and produces an 

 image of the luminous cross upon a screen S'. Mark the dimensions 

 of the image in pencil, and cut it through the card. E is a lens so 

 placed that the card S' is distinctly seen through it. Now remove S' 

 and look through the two lenses at the card S. This arrangement 

 constitutes a telescope. 



" Next let the flame and the eye-lens E change places. Focus the 

 eye-lens on S. The image of the luminous cross in S' is now repre- 

 sented by the cross in S. Remove S and look at the screen S'. This 

 arrangement is a microscope. 



^%^ 



s; 



ILLUSTRATING THE RELATION BETWEEN MICROSCOPE AND TELESCOPE. 



" The focal length of the objective of a microscope is usually very 

 short compared with that of the telescope objective" (p. 357). 



SUBSCRIBERS' NOTICES. 



[These notices will be given six insertions in this column at 25 cents per line or fraction thereof.] 



WANTED.— Vol. VII. No. 2 (Feb., li 



), of American Monthly Microscopical Journal. 



A. G. YOUNG, State House, Augusta, Me. 



WANTED. — Diatom material from America, Australia, and Asia in exchange for fossil and recent 

 material from Denmark. 



CHR. MICHELSEN, 33 Nedergade, Odense, Denmark. 



FOR EXCHANGE. — Slides of diatoms from peat bogs in Boston, Mass. 



F. F. FORBES, Brookline, Mass. 



WANTED. — Vol. I, American Monthly Microscopical Journal. 



C. K. WELLS, Marietta, Ohio. 



WANTED TO PURCHASE. — Histological slides from normal tissues of mammals, prepared with 

 a view to show the size of the " cells," in cases where the age of the animal-subject is definitely known, 

 as bearing on the question of a supposed diminution in the size of cells in aging organisms, correspond- 

 ing to a similar decrease, reported by Balbiani and Maupas in aged unicellular life. Address, 



C. A. STEPHENS' LABORATORY, Norway Lake, Maine. 



WANTED. — Diatomaceous material of good quality, fossil and recent. I offer diatom slides or 

 photo-micrographs in exchange. 



D. B. WARD, Poughkeepsie, N. Y. 



OFFERED. — Diatomaceous Earth from Utah (Desert) for Histological Mounts. 



PROF. ORSON HOWARD, Salt Lake City, Utah. 



