SCIENTIFIC SUMMARY. 
129 
the method employed by M. Marquier in their production. Mr. Henry 
Swan described a new and very ingenious apparatus which he has devised 
for optically uniting a pair of stereoscopic portraits, and so producing an 
image apparently solid or in relief. The principle of its construction 
may be gathered from the following extract from Mr. Swan’s account of 
his instrument : — 
“ The union and combination of the two flat pictures (transparencies 
upon glass) so as to produce the true stereoscopic image, I manage by 
taking two rectangular prisms of flint glass or crystal, ground to an angle 
of about 39° or 40°. These two prisms I place together, with their widest 
sides in contact, or nearly so, so as to form one solid quadrangular prism, 
divided lengthwise by a thin film of air. Now, if I stick a blue wafer on 
one side of this combination, and a red one at the back, I shall find that 
when I look straight at this combination, the rays which reach one of my 
eyes are those which are transmitted directly through the prisms, so that 
with this eye I see the object which lies at the back. On the contrary, 
the rays which reach the other eye are not from the back, but from the 
side only, as they are reflected from the surfaces of the prisms where they 
touch each other. Hence, with this eye we see by reflection the object at 
the side, which, however, appears to be at the back, like the other 
In fact, if we draw a line perpendicular to the surface of the prism next 
the eye, all the rays which fall on one side of that line will suffer total 
reflection from the inner oblique surface of this prism, while nearly all 
the rays which fall on the other side of this line will be transmitted direct 
through the two prisms.” 
In consequence of the images of the red and blue wafers being in this 
way separately presented to the eyes, it results that but one image is 
apparent, as in the ordinary stereoscope, and this assumes a purple hue, 
as the consequence of the red and blue images being superposed on each 
other ; and if, instead of the coloured wafers, we substitute the two 
slightly dissimilar pictures which constitute the stereoscopic pair, we 
obtain, of course, the image apparently solid. A certain amount of skill 
is requisite in mounting the transparent pictures, so as to ensure their 
precise coalescence, or the eye may become distressed during their examina- 
tion through the instrument. 
The specimens exhibited by Mr. Swan at the meeting of the British 
Association were mounted in morocco cases or in gold, to be worn as 
charms. Some of them were coloured, and all were backed by opal glass, 
to assist in diffusing the light. The effect on looking through them was as 
pleasing as the contrivance is novel and ingenious. 
Dr. T. L. Phipson, in a communication to the “ Moniteur de la Photo* 
graphie,” explains the fact that photographic collodion usually becomes 
reddened shortly after being iodized, by assuming tbe separation of free 
iodine, from the salts of potassium or ammonium commonly used, to be 
the direct consequence of the decomposing action gradually exerted by 
these alkaline compounds upon the dissolved pyroxyline, the hyponitric 
elements of which are said to be withdrawn much in the same way as by 
the operation of a caustic alkali. Collodion which has been rendered 
sensitive by the iodide of cadmium is known to be much less liable to 
change ; probably in consequence of this salt being perfectly neutral, and 
therefore unable to remove the acid constituents from the gun-cotton ; 
VOL. IIT, — NO. IX, K 
