120 Progress in Science. (January, 
of the sides of these prisms being made of plane and parallel glass, crown 
glass prisms were cemented on to them in the manner shown in the diagram 
(Fig. 9), where A represents the fluid prism and BB the crown-glass prisms 
added on either side. In a paper read before the Royal Society by 
Mr. Gassiot, April 7th, 1864, he says :—‘‘ In place of giving to the fluid prisms 
two pairs of parallel sides, advantage has been taken by Mr. Browning of the 
difference between the refractive and dispersive properties of crown glass, 
having a refracting angle of 6°, these have been substituted for one of the outer 
plates of each prism, the bases of these crown glass prisms being brought to 
correspond with the apex of the fluid prism. By this means the angle of 
minimum deviation of the prisms is so much altered that eleven prisms can 
be used instead of eight. An increase of dispersive ‘power, due to refracting 
angles of 150° of the bisulphide of carbon, is thus gained, minus only the small 
amount of dispersion counteracted by the dispersive power of the crown glass 
prisms in the contrary direction.” In July, 1869, at the suggestion , of 
Dr. Robinson, of Armagh, Mr. Browning made a large dense glass prism of 60° 
into a compound prism, the refracting angle of the dense prism being altered 
to go’. Hehas also made several compound prisms of very dense flint glass and 
Fia. g. Fia. to. 
BB 
light crown glass for the present Earl of Rosse. These compound prisms are more 
expensive than ordinary prisms, even allowing for the extra dispersive power ob- 
tained, and in consequence of the minimum angle of deviation of compound prisms 
being greater, and their length greater, the size of spectroscopes has also to be 
increased, and thus will be rendered more cumbersome. A smaller number of 
compound prisms will produce a given amount of dispersion ; but the number 
of prisms to be made is, under the most favourable circumstances three 
instead of two; and in the case of acircular battery the smaller number of 
prisms will occupy a larger circle. The number of plane faces is also greater 
for a given amount of dispersion, the practical difficulties in securing accuracy ° 
not being diminished by the fact that four faces out of the six in a compound 
prism are connected together. On the score of saving light, such prisms 
possess undoubtedly some advantage. Mr. Browning has made for Mr. 
Rutherford, of New York, a compound prism of glass, on a plan which seems 
to possess some advantages over any hitherto used. The diagram (Fig. 10) 
represents this prism. In this diagram the two darkly shaded prisms are of 
dense flint of go’, while the three other prisms are of crown. Such a prism is 
very nearly equal in dispersive power to three ordinary flint glass prisms of 60°. 
There is no loss of light at the two intermediate surfaces, and it is much more 
compaé. Experiment has proved that the angles of the flint glass prisms in 
this arrangement cannot with advantage be made more than go’, nor the 
outside crown glass prisms less than 30°. 
Microscopy.—A microscope lamp, combining most of the advantages of 
existing ones, has been contrived by a committee of the South London 
Microscopical and Natural History Club, for use at its meetings. It is 
mounted on a stand supported ona solid ring, after the retort stand model, 
which experience has proved to be less liable to overturn than any other. 
The lamp revolves in its socket, so that either the edge or the flat side 
of the flame can be used at pleasure. MHailes’s porcelain shade has 
been adopted, which has the double advantage of protecting the eyes from the 
glare of the lamp as effectively as a metal chimney without its disagreeable 
heat, and affording an easy means of obtaining white cloud illumination. The 
burner is a small one of the best American manufacture; and all the parts are 
so simplified that the lamp can be supplied at a lower cost than the majority 
of microscope lamps, which it quite equals for all practical purposes. 
