Intelligence and Miscellaneous Articles. 243 



I have sought, therefore, a means of estimating the degree of 

 transparence of the air with ease and exactness ; and in this investi- 

 gation I have been materially aided by Professor Thury of Geneva. 

 Under his directions and by his plans has been constructed, by the 

 Genevese Society for making physical instruments, the photometer 

 for measuring variations in the transparence of the air, of which the 

 following is an abridged description. 



The instrument renders it possible to make a simultaneous com- 

 parative observation of two similar views placed at different dis- 

 tances. The difference between the optical images of these views 

 expresses the effect produced by the interposition of a layer of air of 

 known thickness equal to the difference in the distance of the two. 

 The two images are made equal by means of an arrangement fitted to 

 the instrument itself ; and the measure of the modification necessary 

 to bring about this equality gives the difference in the brightness of 

 the two images, and hence the effect of the interposition of the 

 layer of air. 



The two objects to be compared should be viewed by the same eye, 

 in the same conditions, and in the same general direction ; and since 

 a comparison of objects can only be made by that of their images, 

 these should be quite like the objects they represent. Moreover 

 any light foreign to the objects should be carefully excluded. 



It has been attempted to realize these conditions by means of two 

 telescopes, each having an object-glass, but with an eyepiece common 

 to both. Each object-glass gives its image in half the field of the 

 eyepiece. The optical axes of the two object-glasses form an angle 

 which may vary from zero to * 29° at the will of the observer. 

 The luminous ray from the principal axis of each object-glass is 

 brought into a direction parallel to the axis of the eyepiece by two 

 successive total reflexions which it undergoes ; the first takes place 

 in a moveable prism, and the second by a fixed one placed very near 

 the focus of the eyepiece. The angular motion of the first prism is 

 connected with that of the moveable part of the corresponding tele- 

 scope, so that the angle described by the prism is always half that 

 of the telescope. Thus whatever be the point to which the telescope 

 is directed, the image of this point always occupies the centre of the 

 eyepiece. What takes place with the luminous beam in one of the 

 telescopes takes place in exactly the same manner with the other ; so 

 that in the focus of the eyepiece there are two juxtaposed images. 



On the other hand, the motion of a milled head placed in the 

 hand of the observer makes the telescope describe equal angles with 

 the axis of the eyepiece, which is that of the system ; and thus the 

 two images are produced in identical conditions as regards reflexions 

 in prisms. In order that the observer may at any time be certain 

 that there is complete identity in the manner in which the two 

 images are produced, the whole apparatus is capable of being turned 

 by an angular motion of 180° about the common axis of the system. 

 There is thus seen with one of the telescopes what was previously 

 seen with the other. When there is a difference between the two 

 images, the observer may easily convince himself that it is not 

 owing to the instrument itself; or if there are small differences 



