492 SUMMAKY OF CUKKENT RESEARCHES RELATING TO 



Dr. V. Chiusoli points out * that tlie conclusions of Guebliard can be 

 verified by a simple experiment. 



Using tlie strongest eye-piece and the weakest objective, focus the 

 Microscope upon a coarse object of sharp outline (e. g. hairs). Then, 

 according to Guebhard, the virtual image formed by the eye-piece is at the 

 punctum remotum of the eye. Next move the tube suddenly towards the 

 object through a fraction of a millimetre by means of the micrometer- 

 screw; the object at first appears blurred, but after a short effort the 

 details will reappear with their former distinctness. The image in this 

 case has been brought nearer to the eye, and can only be seen clearly again 

 after an effort of accommodation. The movement of the tube must be small, 

 since it will correspond to a large displacement of the image. 



In the same way, if the vision be suddenly transferred from one part to 

 another of the same object without any movement of the tube, an effort of 

 accommodation will be necessary, since the different parts of the object do 

 not lie in the same focal plane. 



These facts indicate the correctness of Guebhard's conclusions and the 

 error of the impression that the virtual image is always at the least distance 

 of distinct vision.f 



Care of the Eyes in Microscopy.t — Prof. S. H. Gage recommends the 

 microscopist (in addition to keeping both eyes open and using an eye 

 screen if necessary) to " divide the labour between the two eyes, i.e. use 

 one eye for observing the image awhile and then the other." 



He considers that " witli a Microscope of the best quality and suitable 

 light — that is, light which is steady and not so bright as to dazzle the eyes, 

 nor so dim as to strain them in determining details — microscopic work 

 should improve rather than injure the sight." 



Keeber, a. — Bestimmung der Farbe, fiir welche die spharische Aberration zn heben 

 ist. (Determination of the colour for which the spherical aberration is to be 

 corrected.) 



[The author inquires how the aberration should be corrected so that the average 

 spherical aberration of all colours shall equal O, due regard being had to their 

 different intensities ; and concludes that this condition is secured when the 

 correction is made for light of wave-lengtii 0" 00055, that is, for a ray lying 

 between D and E. It appears, therefore, that this resixlt is practically realized 

 by the correction as it is ordinarily made.] 



Ccnfral-Zlg. f. Uptik u. Mech., VIII. (1887) pp. 49-51. 

 Nelson, E. M. — Microscopical. 



[Reply to queries on optical tube-length ; tests for spherical aberration in objectives, 

 with remarks on the fallacy of the American sj'stem of testing ; stiiges. &c.] 



/,>//. .Vech., XLV. (1887) p. 221. 

 RoTSTON-PiGOTT, G. W. — Microscopical Advances. XVII. 



[Diffraction, Ancient and Modern.] Engl. Mech., XLV. (1887) p. 93 (1 fig.). 



Zech, p. — Elementare Bebandlmig von Linsensystemen. (Elementary treatment of 



Lens-systems.) 8vo, Tijbingen, 1887. 



* Rev. Scientif., 1884, p. 62. Cf. Zeitschr. f. Wiss. Mikr., i. (1884) pp. 558-9. 



t Prof. C. M. Gariel (Rev. Scientif., 1883, p. 789 ; Central-Ztg. f. Optik u. Mech., v. 

 (1884) pp. 218-9, 3 figs.) gives an elementary proof of Gue'bhard's results, showing 

 that if the focus lies behind the nodal point the magnifying power increases as the image 

 approaches the eye, and is greatest at the punctum proximum. If the focus is in front of 

 the nodal point tlie magnifying power increases us the image recedes, and is greatest at 

 the pttnctum remotum. If the locus coincides with the nodal point the magnifying power 

 remains constant. Cf. on same subject, Monoyer; Comptes Rendus, xcvi. (1883) 

 pp. 17S5-7; Central-Ztg. f. Optik u. Mech., v. (1884) pp. 217-8. 



X ' Notes on Microscopical Methods,' 1886-7, pp. 8-9. 



