Proceedixgs of the Polytechnic Association. 761 



to powder, by a very rapidly revolvinp; cutter. The great lieat 

 generated sets tlie particles of metal on lire, and after scintillating, 

 they fall down in a reddish brown dust, which is gathered, placed in 

 a crncible, and melted. .When cooled this melted dust is found to be 

 ingots of very good steel. It is claimed that the carbon is all bnrned 

 out, which is, probably, true ; but the act of burning has produced 

 an oxyd of iron, in other words, reduced the iron again to ore. If 

 the process is modified so as to produce good steel, still it must be 

 far more expensive than that of Bessemer, or the ordinary mode of 

 makino; steel from wrouo^ht iron. 



The Microscope. 



A correspondent of The American Naturalist thus explains the 

 terms by which the power of the microscope is designated : "English 

 and ximerican opticians name their objectives (i. e., the lens or lenses 

 placed next to the object, that next to the eye being the eye-piece) 

 from their magnitying power ; thus a one-fourth inch objective has 

 the same power as a simple lens of one-quarter inch focus. Conti- 

 nental European makers generally distinguish their instruments by 

 numbers, the highest number indicating the highest power ; but as 

 each maker has his own system, the actual power of the instrument 

 must be ascertained by trial. Instruments also diifer from their 

 names, and they cannot generally be depended on. The theoretical 

 power of the microscope is measured from an arbitrary standard o f 

 ten inches ; thus, one inch is said to magnify ten diameters, a quarter 

 inch forty diameters. If the standard is taken at five inches, as it is 

 by some, then the ' power ' is one-half as much. The power of the 

 microscope is that of the objective multiplied by that of the eye- 

 piece ; if the objective magnifies ten diameters, and the eye-piece ten, 

 the result is 100 diameters. Angular aperture is the angle in the 

 surface of the front lens, at which light will enter the objective ; the 

 greater the angular aperture, the more light, and usually the greater 

 the resoh'ing power. An amplifier is an achromatic combination 

 inserted in the compound body of the instrument to increase the 

 power of the objective and eye-piece. Immersion lenses have lately 

 attracted great attention, though they were made by Amici many 

 years since. The objective is immersed in water ; that is, there is a film 

 of water between the front of the objective and the object, on the 

 thin glass covering it. The efiect is a great increase of light and 

 better definition." 



