TRANSACTIONS OF THE SECTIONS. 15 
mere efforts of the will can easily bring either the one or the other into view. The 
importance of this law, which enables the mind to select its image, was pointed out 
in different cases of ordinary vision. It obviates the difficulty already adverted to, 
of having two different pictures on the same spot; it has not improbably an import- 
ant influence in producing the general stereoscopic effect; it also, to some extent, 
remedies the effect of squinting, by obliterating the picture in the imperfect eye, 
which could not be else done without sbutting it. The effect of the law, in some 
extraordinary cases, was also noticed, especially in the power of the will to fix images 
on the sight, as Sir Isaac Newton instances in his own case (see his ‘ Life,’ by Sir 
David Brewster). The author pointed out the great interest of the subject, not only 
in its practical aspect, but also as having an important bearing on the connexion be- 
tween mind and matter. 
On some Optical Properties of Phosphorus. 
By Dr. Giavstone and the Rev. T. P. DAtE. 
The authors had recently examined the effect of temperature on refraction by 
means of the instrument constructed by the Rev. Baden Powell, from a grant of the 
British Association. They had extended their examination of the optical properties 
of phosphorus to other points. Solid phosphorus at 25°C. gave the following 
refractive indices :— 
IBRXGG AGIAN fe Ua ie a eg es wel SRODG 
re spre alegre eerame et Too. LA 
HictieMmenviOlet Fo tuives Ween eg) «ea iSOOT, 
This shows an amount of refraction only just exceeded by diamond or chromate of 
lead, and an amount of dispersion perhaps unapproached by that of any other sub- 
stance. 
Melted phosphorus at 35° C, gave the indices— 
RISGU Ine AY ter. cgay Yess oe oe St OSSO 
aa iy’ 9 fie ee rill ye rn Ac an inca Nae ALO 7 /221(6) 
fe HEN te ae re VOL OO 
Figs 25 pga Soe Rade alt Sate nae Cr | 
Extreme violet . 2°2267 
This shows a considerable diminution both of the refractive and dispersive power. 
At the temperature of 35° C., the refraction of the orange ray through solid phos- 
phorus was to that through the liquid body in the ratio of 2°117 to 2-071. 
The change of refrangibility caused by change of temperature is greater with liquid 
phosphorus than with any other known body. A saturated solution of phosphorus 
in bisulphide of carbon is almost as refractive and dispersive as the element itself. 
Refractive indices for the principal fixed lines of the spectrum, as seen through such 
a solution, were given. ‘There is a certain indistinctness about the prismatic image 
seen through phosphorus not depending on opacity or crystalline structure, and not 
arising from the high refraction or dispersion, nor wholly from the great sensitive- 
ness to changes of temperature. It may be connected with the well-known allotropic 
variations of this body, no particular specimen being really homogeneous. The 
phosphorus experimented on was colourless ; yellow phosphorus cuts off the red rays. 
The white flame of phosphorus contains all the visible rays of the solar spectrum, but 
exhibits no trace of any dark line or band. 
A Hand Heliostat, for the purpose of flashing Sun Signals, from on board 
Ship or on Land, in Sunny Climates. By F. Garton, Sec. R.GS. 
A flash of sunlight from a looking-glass, of a few inches in the side, can be seen 
assist in making it apparent. Let a person stand before a papered wall with pictures hanging 
on it, and shift the optic axes so as to bring different patterns of the paper together. The 
images of the pictures formed in the two eyes will then be separated in the sensorium, and 
shifted off upon the image of a part of the paper; and if the attention is turned upon these 
parts, it will be found that it may be made to depend on an effort of the will whether the pic- 
ture or the paper will be seen there. The experiment might be still better made with a suit- 
able piece of a different paper pinned on the wall; as in the case of pictures, the more inter- 
esting or much brighter objects are apt to force themselves too much on the attention. 
