Experimental Contributions to the Theory of Heredity. 24^ 



both these conditions are fulfilled that perfectly sharp images of 

 spectral lines extending from the red to the high zinc line 2138*30 can 

 be photographed on the same surface. Celluloid films are used, glass 

 not being sufficiently flexible. 



Using the middle position showing the whole spectrum on a plate, 

 the angle is 40 3 , and the curvature is 190 mm. radius. 



The condensers are of quartz, and are piano-cylindrical — one being- 

 double the focus of the other. The object of this, when spark-spectra 

 are being photographed, is to concentrate on the slit a line instead of 

 a point of light, as would be the case if ordinary lenses were used. 



^lien photographing phosphorescent spectra — or, in fact, any spectra 

 the wave-lengths of which are either unknown or require verification 

 — I always photograph on the same film a standard spectrum, usually 

 of an alloy of equal molecular weights of zinc, cadmium, tin, and 

 niercury. This forms a hard somewhat malleable alloy, giving 

 throughout the whole photographic region lines the wave-lengths of 

 which are well known. The chief objection to this alloy is its volatility 

 — the poles requiring frequent adjustment. Recently I have used 

 pure iron for this purpose \ this has the advantages of giving a great 

 number of fine lines whose wave-lengths are accurately known, and not 

 being very volatile, the poles do not rapidly wear away. If the poles 

 are kept about 1 mm. apart there is little or no interference from 

 air lines. 



The most simple method of applying the standard lines to an 

 unknown spectrum is by the successive employment of two slightly 

 overlapping diaphragms immediately behind the slit, one being used 

 for the experimental and the other for the standard spectrum. In this 

 way, without disturbing the instrument, the two spectra can be recorded 

 on the plate one over the other ; the overlap of 1 mm. being in the 

 optical centre of the train. The resulting negative is then transferred 

 to a micrometer measuring machine of special construction, having a 

 screw of 1 100 of an inch pitch, and a means of accurately determining 

 1 1000 of its revolution — thus measuring directly to the hundred- 

 thousandth of an inch. In this way, in a five-prism spectrograph 

 having lenses 700 mm. focus, it is possible to determine wave-lengths 

 of photographed lines to the sixth figure. 



Experimental Contributions to the Theory of Heredity. A. Tele- 

 gony/' By J. C. Ewaet, M.D., F.E.S., University of Edin- 

 burgh. Eeceived May 29 — Bead June 1, 1899. 



I. Introductory. 



The belief in telegony, or what used to be known as the " infection 

 of the germ " or " throwing back " to a previous sire, has long prevailed 



