38 



THE AMERICAN MONTHLY 



[February , 



the ^ was a mistake ; I meant just what 

 I said, this ^^ periscopic has wonderful 

 definition. 



But it was not of this that I intended 

 to write ; so much has been said about low 

 and high angles, and penetration, that I 

 am inclined, is an ex-cathedrd style, to 

 give my own ideas upon these questions, 

 founded upon my own experience. 



First. — Rays of light are radiated, for 

 a space of 180° from each minutest point 

 of an object, that point forming the centre 

 of the circle. 



Second. — The rays taken into the ob- 

 jective and transmitted to the eye, pro- 

 ceed, so to speak, like a shower of im- 

 pulses, to the optic nerve, and if enough 

 of such rays, from each point, act upon 

 the optic nerve, a sensation of sight, and 

 form, or intimate definition is produced. 



Third. — This shower of impulses is 

 strong or weak in proportion to the num- 

 ber of such rays taken into the objective 

 and transmitted. 



Fourth. — The larger the angle, in pro- 

 portion to the power of the objective, the 

 stronger the impulse, because of the larger 

 number of rays from each point, reaching 

 and acting upon the optic nerve. (By this, 

 I do not mean a glare of confused rays.) 



Fifth. — In low powers, since a large 

 actual field is taken in, very high angles 

 are not needed for ordinary definition, 

 such as might be needed with greater am- 

 plification. 



Sixth. — In the high powers, having 

 great amplification, definition of each 

 minutest point of the object, is what we 

 seek, and therefore we must have as many 

 of these radiated (not confused) rays as 

 we can. Then, we must have high angles, 

 in order to have our shower of impulses 

 strong, and produce the necessary impres- 

 sion, upon the optic nerve, of the slighest 

 differences of light and shade which de- 

 fine and give us the idea of form. 



Seventh. — Up to >^-inch, mpderate 

 angles will give us all needful definition 

 for rapid work. 



If we wish to see details, we will require 

 higher angles, accordingly. 



For both high and low powers, there 

 are three classes of well-corrected object- 

 ives, the low, moderate, and high angles, 

 each of these to be used according to our 

 work. 



Each of the above classes should be of 

 best quality and corrections, and we find 

 in low and high powers that any objective 

 of either class may be a first-class object- 

 ive, whether the angle be low, moderate, 



i: 



or high. Of course we might make mo: 

 classes, but the three cover the ground b/ 

 extending each class over a number of 

 degrees, until that class meets the next 

 higher in angle. 



Resolution, in perfection, is only found 

 in highest angle ; perfect correction, or th 

 manner of correction, is not the same fo 

 each class. This, it would take too mucl 

 time to explain, and I leave it, but will 

 instance, resolving objectives, which mus 

 be under-corrected, chromatically, to ge 

 best resolution. 



Every objective must be corrected aC' 

 cording to its angle power, and the use to 

 be made of it, and yet each and every 

 class, so corrected, be perfect in its correc- 

 tions, for the use intended. 



Penetration goes hand in hand with 

 moderate angles and perfect correction, 

 and is not the child of faulty corrections. 



Best penetration is found in first-class 

 objectives of the lower powers, with mo- 

 derate angle. Penetration exists in all 

 powers of moderate angle, where perfect 

 correction exists. 



As to resolution and penetration, the 

 reasons for the quality, existence and 

 extent of each, would require more room 

 than this article allows, I may give them 

 at some future time. 



S. A. Webb. 



OWSEGO, N. Y., Jan., 1881. 



NOTES. 



— The fifth edition of Mr. Zentmayer's 

 Catalogue of Microscopes, contains illus- 

 trations of some of his instruments that 

 were not in the former one. Mr. Zent- 

 mayer now makes five styles of micro- 

 copes, respectively known as "Centennial," 

 " Army Hospital," " New Model Army 

 Hospital," " Histological," which is now 

 made as a binocular also, and finally a 

 " Student " stand, for which he charges 

 only $38.00. Besides these he has a 

 botanical dissecting microscope, and a 

 pocket instrument. 



— Mr. Fr. J. Emmerich, of this City, is 

 now prepared to execute orders for the 

 microscopes and objectives of Mr. Carl 

 Zeiss, of Jena. He expects, before long, 

 to receive a stock of the most salable 

 goods manufactured by Mr. Zeiss, so that 

 he can fill orders promptly, and at the 

 lowest possible price. He can now fur- 

 nish catalogues in German, and will have 



