OF SOME OF THE STAES AND NEBULJE. 
535 
reflected down the axis of the first telescope, traverses the prisms in succession, enters 
the second telescope, is reflected by the mirror at its focus, and emerges from the tele- 
scope parallel to its direction at incidence ; it then traverses the prisms in the reverse 
order, and is brought to a focus at the cross lines of the first telescope. 
If the deviation of the rays in passing through the prisms from east to west differs from 
that produced during their passage from west to east, the image of the vertical spider- 
line formed by the rays which have traversed the prisms twice will not coincide with 
the intersection of the spider-lines as before. 
I have found, however, that when the instrument is properly adjusted, the coincidence 
is so perfect with respect to rays of all refrangibilities, that the image of the vertical 
spider-line is seen with perfect distinctness, though the rays which form it have passed 
twice through three prisms of 60°. 
If we observe the coincidence of this image with the intersection of the spider-lines 
at the focus when the rays pass through the prisms first in the direction of the earth’s 
motion and return in the opposite direction, we may then reverse the whole instrument, 
so that the rays pursue an opposite path with respect to the earth’s motion. T have 
tried this experiment at various times of the year since the year 1864, and have never 
detected the slightest effect due to the earth’s motion. If the image of the spider-line 
is hid by the intersection of the cross lines in one position, it remains hid in precisely 
the same way in the other position, though a deviation corresponding to one-twentieth 
of the distance of the components of the line D could be easily detected. 
On the other hand, M. Fizeau* has observed a difference in the rotation of the plane 
of polarization according as the ray travels in the direction of the earth’s motion or in 
the contrary direction, and M. Angstrom has observed a similar difference in phenomena 
of diffraction. I am not aware that either of these very difficult observations has been 
confirmed by repetition. 
In another experiment of M. Fizeau, which seems entitled to greater confidence, 
he has observed that the propagation of light in a stream of water takes place with 
greater velocity in the direction in which the water moves than in the opposite direc- 
tion, but that the acceleration is less than that which would be due to the actual velo- 
city of the water, and that the phenomenon does not occur when air is substituted 
for water. This experiment seems rather to verify Fresnel’s theory of the ether ; but 
the whole question of the state of luminiferous medium near the earth, and of its 
connexion with gross matter, is very far as yet from being settled by experiment. 
June 10, 1867. James Clerk Maxwell. 
§ II. Description of Apparatus. 
All the experiments were made with my refractor by Alvan Clark, of 8 inches aper- 
ture and 10 feet focal length, which is mounted equatorially, and carried very smoothly 
* Ann. de Chimie et de Physique, Feb. 1860. 
4 E 
MDCCCLXVIII. 
