Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
The Sensations of Vision 
[163, 164. 
the object just vanished. Instead of horn-plates, de Limency and Secretan1 
used paper discs. The other instrument is the lamprotometer, proposed by an 
unknown person2, for measuring the brightness of daylight. The method 
consisted in finding the strength of a litmus solution required to cause a 
platinum wire illuminated by daylight to disappear when it was viewed 
through a glass cell filled with this substance. The limit of sensitivity of the 
eye is, however, too indefinite for measurements of this kind, and the errors 
might be three or more times as great as the magnitudes concerned. A pho¬ 
tometer designed by Albert3 and another one by Pitter4 depend on the 
same principle. 
On the other hand, there were two other ways along which the more 
perfected methods now in use were gradually developed. One of these ways 
was intended for measuring the brightness of stars. By inserting a diaphragm 
in front of the telescope, Sir. J. Herschel reduced the aperture of the instru¬ 
ment and thus diminished the amount of light coming from the brighter star 
at which the telescope was pointed. A. v. Humboldt’s astrometer is based 
on the same principle. This is an ordinary mirror sextant. The telescope in 
the instrument is pointed towards a mirror, one half of which is silvered and 
the other not, one star being seen directly through the unsilvered portion and 
the other star by means of the silvered portion and another mirror. By shifting 
the telescope at right angles to the dividing line between the two halves of the 
mirror, more rays will be received from one star and fewer from the other, 
and thus the images of two stars or the two images of one star can be made 
equal or unequal at will, and the intensities of the light compared in the two 
cases. The advantage of Humboldt’s method is that the two stars to be 
compared are seen close together in the field of the same telescope. But the 
comparison of such small intense point-sources of light is more difficult than 
the comparison of bright surfaces. This fault is remedied in Steinheil’s 
objective photometer.5 This is a telescope with its objective divided in half. 
In front of each half there is a reflecting right-angle glass prism. The instru¬ 
ment is so adjusted that the observer sees one of the stars to be measured 
through one half of the objective, and the other star through the other half. 
Then the two halves of the objective are each shifted a little so that the 
images of the two stars are blurred and no longer distinct; the intensity of 
illumination being diminished in proportion as the areas covered by the 
images are increased by shifting the two halves of the objective. Each half 
is provided with a rectangular diaphragm which may be exchanged for another 
one of a different size. When the adjustment is right, the two images of the 
stars appear juxtaposed as two large rectangles of nearly the same size and 
of equal brightness, so that the conditions are the most favourable for detect¬ 
ing any small differences of brightness. This was the first instrument that 
enabled us to make accurate measurements of the light of the fixed stars and 
planets. Schwerd6, on the other hand, used diffraction effects of small cir¬ 
cular diaphragms to produce bright surfaces. 
But for researches in physics, where we are concerned with ascertaining 
how much of the light has been lost on the way by refractions, reflections and 
other adventures, a good way of reducing the more intense light is by re- 
1 Cosmos. VIII. 174.—Polyt. Zentralblatt 1856. 570—Dinglers polyt. Journ. CXLI. 73. 
2 Poggendorffs Ann. XXIX. 490. 
3 Dinglers polyt. Journ. C. 20 and Cl. 342. 
4 Mechanics Magazine. XLVI. 291. 
5 Poggendorffs Ann. XXXIV. 646. — Denkschriften der Münchener Akad. Math.-phys. 
Klasse. Bd. II. 1836. — Johnson’s method in Cosmos. III. 301—305 is similar. 
6 Bericht über die Naturfor scher ver Sammlung 1858.


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