1 8 SECTION PHYSICS. 



Pillischer, in which the prisms can be pushed on one side, and the 

 object seen through the slit without taking off the prisms. These 

 spectrum eye-pieces can be very conveniently used with a binocular 

 microscope, since the natural object can be found and examined by 

 means of one tube, and its spectrum seen by the other tube. 



For the sake of compactness in a travelling microscope, a mov^able 

 slit fitting into the eye-piece like a micrometer may be used, as exhibited 

 by Messrs. R. and J. Beck. 



All these eye-piece methods have the advantage of enabling us to 

 examine the spectra of very minute objects, but the author has found 

 that when they are of moderate size, it is more convenient and less 

 trying to the eyes to make use of the binocular spectrum apparatus, 

 described in his paper in the Proceedings of the Royal Society for 1867, 

 vol. xv., p. 433. The slit and small reflecting prism are placed at the focus 

 of a special object glass of low power, and the direct vision prisms are 

 fixed between the slit and the lens. With this arrangement, it is, how- 

 ever, necessary to insert a cylindrical lens to make the spectrum in 

 focus at the same time as the line of division between the two spectra 

 which are compared side by side. An apparatus of this kind is exhi- 

 bited by Messrs. R. and J. Beck. 



For the measurement of the position of the absorption bands seen in 

 spectra, the author has contrived and regularly used for many years 

 a standard spectrum produced by means of a plate of quartz, cut 

 parallel to the principal axis, placed between two Nicol's prisms, as 

 exhibited by Messrs. Becks, which gives twelve well-defined bands, 

 spread regularly over the spectrum, as described in the paper in the 

 Proceedings of the Royal Society already named. For quick and 

 moderately accurate measurements this plan is very convenient. The 

 chief objection is the difficulty of so preparing the plate of quartz, as to 

 give the bands exactly in one uniform position in every scale. Mr. 

 Sorby, however, proposes that in all cases the position of bands should 

 be expressed in wave-lengths ; and if each observer made a table of 

 wave-lengths for the quartz scale used by him, perfect identity in the 

 position of the bands is of very little importance. 



Mr. Browning exhibits the bright slit micrometer made by him to 

 measure the position of absorption bands, as described in the Montfily 

 Microscopical Journal for 1 870, vol. iii., p. 68. For certain purposes this 



