SCIENTIFIC SUMMARY. 
259 
responds to two-fifths of the distance between Frauenhofer’s lines b and f ; 
the colour of this is green, and though it is bright, it differs from those of the 
nebula by its size. The other two bands were too small and feebly developed 
for Signor Secchi to discover their exact position ; one is near the principal red 
band, and the other is at some distance from the violet one. From these 
facts Signor Secchi concludes that comets are bodies whose constitution is 
closely allied to that of the nebulae, although they differ from them in their 
refrangibility of light. 
Geometric Plans taken by Photography. — The Journal of the Society of 
Arts announces that M. Chevalier, the French optician, has devised an ap- 
paratus for the above purpose. The instrument is provided with a meridional 
telescope and a compass, in order to set it to any given point. A circular 
collodionized glass is placed at the bottom of a camera-obscura formed of 
copper, and moved by clockwork, so as to describe within a given time the 
outer circle, of which the station chosen is the centre, and the various objects 
as they are received in turn by the lens, and photographed on the circular 
plate through an exceedingly narrow slit in the side of the copper box. The 
operation is repeated at their stations, in order to avoid error, and the result 
is said to be highly satisfactory. The thin circular plates are used to lay 
down on paper all the points of the plan described. 
Professor Wheatstone's New Telegraph surpasses everything which the Pro- 
fessor has hitherto accomplished. With the perfected automatic apparatus, 
we learn that six hundred distinctly legible signals may be transmitted in a 
minute. 
Physics under Difficulties. — The Swiss philosophers who took up their resi- 
dences on the pass of St. Theodule, last autumn, for the purpose of making 
meteorological observations during the winter, are all well, and, though 
posted at a height of 10,241 feet, they do not seem to have suffered much 
from cold. 
Relation between Refractive Power and Chemical Constitution. — A very im- 
portant essay on this relationship has been published by M. E. Reichert, in a 
recent number of P oggendorfffi s Annalcn. The paper to which we refer con- 
tains the results of the author’s experiments on solution of common salt of 
different strengths. The percentages of salt, as indiated by the optical 
method and by ordinary analysis, appear to agree very closely. The first 
column of the following table shows the percentages as obtained by analysis, 
and the second gives the results of the optical method : — 
2-26 
7-12 
12-02 
17-25 
23-02 
2-27 
7-13 
'12-07 
17-25 
22-89 
An equally satisfactory result was obtained with solutions of sugar ; but with 
alcohol and acetic acid the differences in the refractive indices are only about 
half as great, and the indications were therefore not so satisfactory. 
How to reproduce Old JAthographs. — Although this subject hardly comes 
within the range of Physics properly so called, it is closely related to it, and 
as the process is both simple and interesting, we lay it before our readers. 
The method, which is a new one, has been described by M. Rigaut. The 
lithograph to be transferred to stone is first laid face uppermost on a surface 
