112 Chronicles of Science. [Jan., 
Photographers will read with interest the announcement by 
M. Prat of the discovery of a compound of silver more sensitive 
to light than the chloride. This chemist has been for some time 
past investigating the chemical constitution of fluorine compounds 
and the isolation of fluorine. M. Prat starts from the fact 
that the fluorides are really oxyfluorides; that the fluoride of 
calcium, for example, is formed of two equivalents of calcrum, one 
of oxygen, one of fluorine; and that, in consequence, the true 
equivalent of fluorine is 29°5, and not 19. In order to obtain 
fluorine, it is only necessary to treat the fluoride of calcium with 
chlorate of potassium, or, what is better, perchlorate of potassium, 
for it is only with this last salt that the reaction takes place. 
Oxygen is disengaged, and a gas is produced, which silver absorbs, 
giving rise to a fluoride of silver, insoluble in water, soluble in 
ammonia, from which it is precipitated by nitric acid, and which is 
altered by the action of light more rapidly than the chloride of 
silver; the formula of the real chloride is Ag Fl, whilst that of 
_ the soluble fluoride of chemists is Ag Fl, Ag O. 
Several new instruments, suitable for the observation of different 
organs of the eye, have been described by M. Robert Houdin. They 
serve also for the examination of entoptic images, or the shadows 
thrown on the retina by intra-ocular bodies. Seven instruments 
of this class have been invented by M. Houdin; these are: 
1, the Iridoscope, for the manifestation of entoptic images; 2, the 
Diopscope, by the aid of which the inversion of the images on the 
retina are determined; 8, the Pupilloscope, demonstrating in a 
magnified form the dilations and contractions of the pupil; 4, the 
Pupillometer, which gives the diameter of the pupil to within a 
quarter of a millimetre; 5, the Diopsimeter, for measuring the 
extent of the field of vision; 6, an Optometer, for the use of any 
persons who wish to determine the distance of distinct vision ; 7, the 
Retinoscope, an instrument with which one can see the vesicular 
group in his own eye. 
In all stereoscopes there is an optical arrangement by which 
the right eye sees an image of one picture and the left eye that of 
another. ‘These images ought to be apparently in the same place, 
and at the distance of most distinct vision. In ordinary stereoscopes 
these images are vertical; the observer has to place his eyes near 
two apertures, and he sees the united images, as it were, behind the 
optical apparatus. Professor J. Clerk Maxwell, F.R.S., has recently 
had made by Messrs. Elliott, Brothers, a real-image stereoscope, 
in which the observer stands at a short distance from the apparatus, 
and looks with both eyes at a large lens, the image appearing as a 
real object close to the lens. The stereoscope consists of a board 
about two fect long, on which is placed :—1, a vertical frame, to 
