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 g-lass plates or 

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

 makes it a most difficult matter to observe objects throug-h 

 a microscope of g"reat enlarg-ing- power. In order to obvi- 

 ate this difficulty and to enable the enlarg-ing- of small 

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

 Paris, Mr. Greenoug-h, constructed a microscope with a 

 g-reater depth of focus, which has since been further im- 

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

 lenses are so arrang-ed that they will permit the inspection 

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

 perfect two systems of lenses are provided, g-iving-the im- 

 age a strong-ly stereoscopic effect. As built by Greenoug-h, 

 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 fpurfold. His theory is that if a certain an- 

 gular aperture is necessary to secure proper definition 

 with a magnifiying 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 



