132 ANNUAL OF SCIENTIFIC DISCOVERY. 



photometer. Others have since been constructed on various prin- 

 ciples, but they are not generally applicable to one of the com- 

 monest problems that occurs in trade, namely, measuring the 

 quality of burning oils by their illuminating power. This, " Ga- 

 lignani " informs us, has now been satisfactorily accomplished by 

 M. Guerard Deslauriers, whose apparatus, which he calls a " luci- 

 meter," consists of two constant-pressure lamps, and a photometer 

 constructed on a new principle. Its shape is triangular; it is 

 made of sheet iron painted black, and varnished, and is divided 

 into two equal compartments. The latter are turned toward the 

 lamps ; the observer stands on the opposite side, which presents 

 nothing but a flat vertical surface pierced with a hole bisected by 

 the partition. Each of the two lamps is so placed as to transmit 

 its light to one only of the two compartments, and exactly to the 

 part where the hole is. The latter is covered with a piece of 

 transparent paper on which, therefore, the rays of light from the 

 two lamps are contiguously depicted. If their intensity is the 

 same, the eye of the observer will perceive no difference ; if there 

 be any, on the contrary, one of the lamps must be brought nearer 

 or removed further off, until the same intensity be obtained. The 

 difference of distance will then mark the relative qualities of the 

 two oils ; which, combined with the quantity burnt in a certain 

 time, is sufficient to determine their marketable value. Mechan- 

 ics' Magazine. 



REAL IMAGE STEREOSCOPE. 



In ordinary stereoscopes the observer places his two eyes oppo- 

 site two lenses, and sees the virtual image of two pictures appar- 

 ently at the same time. In the real image stereoscope of Mr. 

 Maxwell the observer stands about two feet from the instrument, 

 and looks at a frame containing a single large lens. He then sees 

 just in front of the lens a real and inverted image of each of the 

 two pictures, the union of which forms the appearance of a solid 

 figure in the air between himself and the apparatus. 



THE EIDOSCOPE. 



This ingenious invention, recently introduced by Prof. Pepper 

 at the Polytechnic Institution of London, produces the most novel 

 and beautiful effects by means of a mechanical contrivance of ex- 

 treme simplicity. Unlike the kaleidoscope, in which symmetrical 

 effects are evolved by reflection from a number of irregularly- 

 shaped objects, the eidoscope produces geometrical figures of ex- 

 quisite beauty by a simple revolution of two perforated metal 

 discs on their own axes. As the discs are slowly revolved, new 

 and infinitely varied forms are created, with the most delicate gra- 

 dations of tone. It is a remarkable feature that, while these 

 changes are in progress, one perforation, and one only, on the up- 

 per plate, will be found perfectly coincident with another on the 

 lower, while all the rest are irregular and form different combina- 

 tions. When, however, for the magic lantern the discs are put 

 into more rapid motion, the greatly accelerated velocity, instead 



