33 PHYSIOLOGICAL PHYSICS. ichap. xxvi. 



means of the convex lens N. Beyond the slit the 

 tube narrows to the size of the ordinary microscope 

 tube. The rays which have passed through the 

 slit fall first on the lens L, by means of which they are 

 rendered parallel, and in this condition fall on a set of 

 five prisms B. The set of prisms consists of three of 

 crown glass and two of flint glass, those of crown glass 

 being all set with their edges in the same direction, and 

 the two of flint fflass fitting in between the crown- 



O O 



glass prisms, their edges being in the opposite direc- 

 tion. This is shown in the figure, where the shaded 

 prisms are those of crown glass. The effect of this 



combination of prisms is 

 to produce dispersion 

 without deviation ; that 

 is, the light is split up 

 into its elements without 



Fig. 153. Direct Vision I'rism. i i i u 



being bent aside out or 



its straight path. This is shown in Fig. 153. The 

 ray of light falling on the first prism is dispersed 

 and bent to the side. The dispersed rays enter the 

 prism of flint glass, their dispersion is increased, but 

 they are bent in the opposite direction. In the third 

 prism the dispersion is again increased, but the deviation 

 is again reversed, and so on through the five prisms, 

 till the rays leave the last prism with a considerable 

 amount of dispersion, but with their direction similar 

 to that of entrance into the first prism. A spectro- 

 scope with this arrangement of prisms is called a 

 " direct vision spectroscope." This system of prisms 

 is contained in the micro-spectroscope in an inner 

 tube of its own, and can be removed from the tube A 

 or inserted into it at pleasure. In use, the prisms are 

 removed from tube A, and the object on the stage of 

 the microscope focussed, the prisms are then replaced. 

 A screw D permits the collimating lens L to be placed 

 at its focal distance from the slit E, the screw H, as 



