ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 233 



with correction collars are now almost a thing of the past ; cover-glasses 

 of known thickness and the sliding draw-tube having obviated their 

 necessity. The terra " Continental " includes American. 



J. A. L. Sutcliffe* replies to J. W. Ogilvie's remarks, and sums up 

 the whole matter by the question, " AVhich instrument, the English or 

 the Continental, is, by virtue of its design and workmanship combined, 

 capable of affording the scientific worker the greatest facilities for work 

 of a critical character ?" He thinks that an answer by our most eminent 

 workers would not be so much in favour of the Continental type as 

 Osrilvie seems to imasrine. 



o 



Gab EL, C. E. — Microscopy and the Microscopical Examination of Drugs. 



Des Moines. Kenyon Co., 144 pp. (71 figs.) 



(3) Illuminating- and other Apparatus. 



The Biotar, a Projection System with unusually large Aperture 

 and Flat Field. f — ^1. von Rohr gives a description of this lens and an 

 interesting historical sketch of its invention. Although at first sio-ht 

 it might have appeared that the Petzval portrait objective, with arc- 

 light illumination, was capable of answering all requirements, yet there 

 are many instances in which such a light is not available, and where, 

 e.g. cinematograph displays in many private houses, incandescent gas 

 light is the only resource. To the method of correction by which the 

 great merits of projection lenses are usually attained, Abbe, in a paper 

 read before our Society, J gave the name of "allied-correction." But 

 in the same paper he pointed out the value of a system of " independent- 

 correction." He discussed the conditions for correcting the spherical 

 aberrations of the convex lenses, or at least a part of these aberrations, by 

 such other concave lenses as produce chromatic correction, and for this 

 purpose searched for the appropriate means in systems not restricted to 

 the low aperture of telescopic objectives. The general method could 

 be easily indicated as everything appeared to depend on one essential 

 condition, viz. concave surfaces which would introduce negative spheri- 

 cal aberration l)y the difference of the refractive index of consecutive 

 media, as in ordinary binary lenses, but would either exclude chromatic 

 aberration of perceptible amount, or admit chromatic aberration of 

 opposite character. Glass of the necessary kinds was not at that date 

 obtainable, but Carl Zeiss, at Abbe's suggestion, constructed a system 

 with fluid lenses which, at any rate, showed the practicability of the 

 principle of "independent-correction." 



The only other worker in this field appears to have been Charles 

 Piazzi Smyth, who, in 1873, by means of a compound system, com- 

 pletely eliminated the last of the five Seidel errors affecting the strong- 

 light portrait objective. About a year and a half later he published a 

 correction method whereby a system of high aperture yielded a plain 



* Nature, No. 2209 (1912) p. 587. 



t Zeitschr. f. Instrumentenk., xxxi. (1911) pp. 265-70 (5 figs.). 

 X " On New Methods for Improving Spherical Correction, applied to the Con- 

 struction of Wide-angled Object-glass." See this Journal, 1879, pp. 812-24. 



