856 SUMMARY OF CURRENT RESEARCHES RELATING: TO 



for greater distances, but in the case of the smaller it gives too high 

 a reading. 



Tho diameter of the image on tho screen or plate when the eye- 

 pieces 2 and 3 aro used is about 1/5 of the distance of tho image, 

 and with 4 and G ahout 1/3 of that distance. 



The image distance may be reduced in tho case of 2 and 3 to 

 about 400 mm., and in 4 and 6 to about 250 mm., reckoning from the 

 eye-piece. It can bo increasod to any desired amount. 



For purposes of demonstration and for photo-micrography, where 

 small pictures only are required, or in cases where the plate can bo 

 placed at a long distance, the projection eye-pieces of low magnifying 

 power, such as 2 or 3, are to be preferred ; for photographing with a 

 short camera, however, the higher ones should be used. 



It would be too much of an innovation to print the price list in 

 full here, but we may mention that the prices of the objectives rise 

 from 5/. for the dry 16 mm. to 20/. and 22/. 10s. for the homogeneous- 

 immersion of 2*0 mm. and 3*0 mm. and 1-30 N.A., and 25/. and 

 27/. 10s. for those of 1*40 N.A. The eye-pieces vary from 1/. to 2J. 



Prof. Abbe and Dr. O. Schott have also issued a pamphlet * 

 descriptive of the new kinds of glass made at the glass manufactory 

 at Jena, with full details as to their optical and other properties,! 

 and a list of the various kinds, 44 in number, now supplied. The 

 following are most of the novelties in the list: — 



Refr. Index Medium 



for D. I lisp! r.-iuii. 



Light Phosphate-Crown 1-5159 0-00737 



Medium do. 1-5590 0-00835 



Heavy Barium-Phosphate-Crown .. .. 1-5760 0-00884 



Heaviest do. .. .. 1-5906 00922 



Boro-silicatc-Crowu 1-5100 0-00797 



Light Borate- Crown 1-5047 0-00840 



Barium-silieate-Crowu 1-5399 0-00909 



Heavy do. 1-5726 00995 



Heaviest do. 1-6040 0' 01092 



Boro-silicate-Flint 1-5676 0-01216 



Borate-Flint 1-6086 0-01375 



Heavy do 1-6797 001787 



Heaviest Silicate-Flint 1-9626 0-04882 



Suggestions are made as to the glass best suited for various 

 purposes, and on commencing tho perusal of these passages we had 

 the idea that we were coming to a description of the glass used for 

 the new objectives. The following ingeniously worded paragraph, 

 however, closes the subject : — 



" In the case of Microscope objectives which require for the 

 attainment of the highest capacity of performance not only agreement 

 in the course of the dispersion of the crown and flint, but also the 

 correction of the spherical aberration and its chromatic difference, it 

 must be left to the skill of the practical optician to choose the most 

 suitable means from the above series. The new objectives of Zeiss 

 show what can be attained by their practical use." 



* ' Glasschmelzerei fur Optische unci andere Wissenschaftliclie Zwecke. Pro- 

 ductions- und Preis-Verzeichniss,' 20 pp. and 1 pi., 8vo, Jena, 1886. 

 t See this Journal, ante, p. 316. 



