356 Proceedings of Royal Society of Edinburgh. [sess. 
the instrument invariably shows as a more or less badly defined 
dark space between the two spectra ; while in those cases where 
only one light source is employed, one part of the beam being 
absorbed by any given substance and the other part used for 
comparison, the edge of the substance, if the latter be a solid, or 
the meniscus, if it be a liquid, brings about the same result. The 
object of the present paper is to describe an appliance by which 
this difficulty may be overcome. 
The instrument (see fig. 1) is constructed of two separate 
pieces of glass which are cut from the same block to ensure 
Fig 1. 
The two glass blocks cemented The two glass blocks shown 
together. apart as they are before 
being cemented. 
similarity of optical properties. These pieces having been worked 
truly plane on the faces which transmit the light, are silvered 
over the portions shaded in the figure, and are then cemented 
together along their common interface PQRS. The effect of the 
cement, whose refractive index is practically the same as that of 
the glass, is to make the joint nearly optically homogeneous with 
the glass blocks on each side. As will be seen from the diagrams, 
in every case the various faces of the blocks are either perpen- 
dicular, or are inclined at an angle of 45° to each other. 
The glass block thus built up is encased in a metal shell, with 
