ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 117 



: amount of chromatic and spherical aberration for more complete correc- 

 tion by the upper lenses. The first triplet destroys spherical and chro- 

 matic aberration, and the second triplet the secondary spectrum. 



Dr. van Heurck finally tabulates and compares the merits of 3 mm. 

 and 2 mm. apochromats of the most modern makes.* 



Fig. 19 is] reproduced from Czapski's Theorie der optischen Instru* 

 . merited It shows the older form of apochromat, and will be interesting 

 for comparison. It represents an objective of 2 mm. equivalent focus 

 and of N.A. 1*40, on a scale three times the natural size. 



•Charlier, von C. V. L. — Uber akromatische Linsensysteme. (On Achromatic 

 Lens-Systems.) 



[A very succinct article, with more particular reference, however, to tele- 

 scopes.] Ofver. K. VetensJe.-Akad. Forhandl, 1899, pp. 657-669. 



Objectives of Telescopes. A series of articles in the ZeiUchr. f. Iustrumenten- 

 liunde : — 



Harting, H. — Zur Theorie der zweitheiligen verkitteten Fernrohrobjektive. 

 (On the Theory of Cemented Doublet Telescope-Objectives.) 



[An elaborate mathematical article with tables of constants for different kinds 

 of glass.] Dec. 1898, pp. 357-80. 



Wolf, M. — Uber ein Fernrohrobjektiv mit verbesserter Farbenkorrektion. (On 

 a Telescope-Objective with improved Achromatic Correction.) 



Jau. 1899, pp. 1-4 and 1 fig. 



dliJEGH, E. von — Zur Theorie der zweitheiligen verkitteten Fernrohrobjektive. 

 (On the Theory of Cemented Doublet Telescope-Objectives.) 



Feb. 1899, pp. 37-9. 



Harting, H. — Zur Berechnung astronomischer Fernrohrobjektive. (Ou the Cal- 

 culation of Astronomical Telescope-Objectives.) April 1899, pp. 101-11. 



Harting, H. — Uber Astigmatismus und Bildfeldwoibung bei astronomischen 

 Fernrohrobjektiv en. (On Astigmatism and Circular Image Distortion in Astro- 

 nomical Telescope-Objectives.) 



May 1S99, pp. 138-143, with several tables of constants. 



Stetnheil, E. — Farbenkorrektion und spharische Aberration bei Fernrohr- 

 objektiven. (Achromatic Correction and Spherical Aberration in Telescope- 

 Objectives.) June 1899, pp. 177-183 and 3 diagrams. 



Harting, H. — Uber ein astrophotographisches Objektiv mit betrachtlich ver- 

 mindertem sekundarem Spektrum. (On an Astrophotograpliic Objective with 

 considerably reduced Secondary Spectrum.) Sept. 1899, pp. 269-72. 



.Leman, A. — Zur Berschnung von Fernrohr- und schwach vergrossernden Mikro- 

 skop-Objektiven. (On the Calculation of Objectives of Telescopes and of Low- 

 power Microscopes.) Sept. 1899, pp. 272-3. 



Harting, H. — Bemerkung zu dem vorstehenden Aufsatze. (Nute on the foregoing 

 Treatise.) ibid., pp. 274-5. 



* We much regret that in this Journal (1899, p. 337) it was erroneously stated 

 rthat " the form of Jeus figured in this paper is that of the old apochromats when they 

 were^first introduced, and not that of those now manufactuied." — Editor. 

 t Edition 1893, p. 245. 



