716 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Chromatic Correction of Object-Glasses.* — A. E. Conrady, after 

 drawing attention to the utter uselessness of Cauchy's dispersion formula, 

 gives as an alternative 



Next, he explains the method of trigonometrically tracing a ray through 

 a spherical surface, and then shows that if d be the thickness of a lens 

 at the axis, and I) its thickness where traversed by an extra axial ray, 

 and if S n be a small increment of the refractive index n, the equation 

 for an achromatic condition will be 



2 8 n (d - D) = 0. 



Finally, he points out that when a ray is near the axis the angles become 

 so small that in the computation sufficient accuracy is obtained by 

 writing £ (circular measure) 2 instead of the versed sine of the angles. 



(6) Miscellaneous. 



Ultra-microscopic Observations in Solutions of Pure Glycogen.! 

 W. Biltz and Z. Gatin-Gruzewska used the apparatus of Siedentopf and 

 Zsigmondy for their observations on glycogen. Similar observations 

 had been made previously by Raehlmann and others, but the samples 

 of glycogen used by these observers were not pure. The authors used 

 A solutions of glycogen in water and B solutions, to which different 

 reagents were added. The A set showed that in an aqueous solution of 

 glycogen when examined ultra-microscopically there are corpuscles of 

 different sizes ; the size varying with the conditions of the solutions. 

 The B set showed the progressive and regular course of the precipita- 

 tion of glycogen under the influence of increasing quantities of certain 

 precipitants. 



Microscope and Expert Testimony.:}: — A. S. Osborn shows how 

 useful the Microscope is for examining documents, especially in case of 

 fraudulent additions, interlineations and erasures. The paper is fur- 

 nished with excellent illustrations giving examples of retouched writing, 

 forgeries, and lines showing the sequence of writing. 



Reed, L. — The Microscope and food adulteration. 



1'roc. and Trans. Croydon Nat. Hid. and Sci. Soc., 1901, pp. 41-4. 



B. Technique.! 

 (1) Collecting- Objects, including- Culture Processes. 



Detection of Nitrifying Organisms in Sewage Filters. || — The 

 following is the technique employed by Schultz-Schultzenstein (see 

 p. 695) : 100 c.cm. of Winogradsky's nutrient solutions for the nitrite- 



* Monthly Notices Roy. Astron. Soc, lxiv. (1904) pp. 182-S (2 figs.); pp. 458-GO. 



t Comptes Rendus, exxxix. (1904) pp. 507-9. 



X Jonrn. App. Micr., vi. (1903) pp. 2637-48 (8 figs.). 



§ This subdivision contains (1) Collecting Objects, including Culture Pro- 

 cesses; (2) Preparing Objects; (3) Cutting, including Imbedding and Microtomes; 

 (4) Staining and Injecting ; (5) Mounting, including slides, preservative fluids, &c. ; 

 (0) Miscellaneous. 



|| Technology Quarterly, xvii. (1904) pp. 18G-203. 



