98 THE AMERICAN MONTHLY [Mar 



Greenough's Instrument. — The microscope, as con- 

 structed heretofore, only allowed the inspection of even 

 surfaces, and its focus was so limited that objects of any 

 thickness had to be first prepared between glass plates or 

 upon a flat surface to become visible at all. This quality 

 makes it a most difficult matter to observe objects through 

 a microscope of great enlarging power. In order to obvi- 

 ate this difficulty and to enable the enlarging of small 

 bodies as well as of flat surfaces, an American living in 

 Paris, Mr. Greenough, constructed a microscope with a 

 greater depth of focus, which has since been further im- 

 proved by the optician Czapski, of Jena, Germany. Its 

 lenses are so arranged that they will permit the inspection 

 of uneven surfaces, and in order to make the vision more 

 perfect two systems of lenses are provided, giving the im- 

 age a strongly stereoscopic effect. As built by Greenough, 

 the image was inverted, but the present form of instru- 

 ment shows the correct image, the inverted picture being 

 turned around by prisms placed between the lenses, as in 

 the Zeiss field glasses. 



Objective. — Dr. M. C. White of Yale has adopted an ob- 

 jective of 20 mm. focal length, and a numerical aperture 

 of 0.95. It is a magnified copy of a 5 mm. apochromatic, 

 the diameters and radii of curvature of all the lenses be- 

 ing increased fourfold. His theory is that if a certain an- 

 gular aperture is necessary to secure proper definition 

 with a rnagnifiying power of, say, 1,000 diameters, then a 

 similar aperture will be necessary to secure good defini- 

 tion in an image projected on the screen, even if it is ob- 

 tained with a 3-4ths inch objective and a projection eye- 

 piece. 



Pointer. — A convenient pointer for class demonstrations 

 and other work may be made by cementing a human hair 

 to a circular ring of blackened paper or card board which 

 can be placed on the diaphragm or removed at will. 



Color Effects. — -By looking at objects through a screen 

 of two glass plates, one laid upon the other, and one being 

 of a blue tint obtained from oxide of cobalt, while the 



