ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 829 



of a binary system formed by the association of the instrument with a 

 diopter eqiiivalent to the state of accommodation of the eye. 



V. Most authors have treated the influence of an instrument on the 

 visibility of objects by the consideration of magnification ; some by the 

 relative amplifying power ; M. Panum alone has given an expression 

 equivalent to that of the absolute power. The formula of M. Monoyer 

 has the advantage of being more simj)le, of being applicable to all optical 

 instruments, and of taking account of all conditions of distance, of the 

 instrument, of the object, and of the accommodation. 



VI. The relative amplifying power becomes equal to the dioptric 

 power only under two circumstances ; when the distance of accommo- 

 dation is infinite, whatever the distance of the instrument from the eye, 

 or when the second focal point coincides with the first nodal point of 

 the eye, whatever the distance of accommodation. 



VII. The dioptric power can then serve to measure the power of the 

 instrument. 



VIII. The dioptric power of an instrument situated at an invariable 

 distance from the eye is obtained by dividing by the displacement given 

 to the object the difference of the two magnifications which result from it. 



IX. The dioptric power of a system of which the second focus 

 coincides with the first nodal point of the eye is equal to the quotient of 

 the magnification by the distance of accommodation. Thence follows a 

 very simple method for experimentally determining the dioptric power. 



X. The determination of the cardinal points of a centered dioptric 

 system, hitherto obtained by the application of the formula of conjugate 

 foci, is advantageously obtained by the aid of the formula of magni- 

 fication. 



XT. This determination can be effected by the aid of simple apparatus 

 without making very important errors. It would be facilitated if instru- 

 ment-makers would furnish the micrometer-screw with a graduation. 



XII. Every Microscope offered by a maker ought to be accompanied 

 by the optical constants most accurately ascertained, which alone deter- 

 mine the value of the instrument. 



KoYSTON-PiGOTT, G. W. — Microscopical Imagery. 



[BriUiant miniatures and minute molecules — Colias Ccesonia.'] 



Journ. of Microscopy, II. (1889) pp. 205-9 (1 pi.). 



(6) Miscellaneous. 



The late Chas. Fasoldt.* — The following obituary notice is from 

 the pen of Prof. W. A. Rogers. 



" MicroscopistS will hear of the death of Mr. Fasoldt with uhfeigned 

 regret. The work which he has done in fine rulings and in micrometry 

 entitles him to a better recognition than he has received. While there 

 may be a difference of opinion in regard to his skill in the production of 

 test- plates, as compared with Nobert, it must, I think, be admitted that 

 he has made some j)lates which are quite as good as the best of Nobert's. 

 When it is remembered that he must have been more than fifty years of 

 age before he took up the problem of micrometric rulings, and that he had 

 had no previous knowledge of the subject, his success has certainly been 

 most remarkable. 



Two circumstances have acted as a hindrance to the recognition to 



* The Microscope, ix. (1889) pp. 174-5. 



