PRISMS FOR OBLIQUE ILLUMINATION. 137 



denser of this description lie states to be suitable also for all ordi- 

 nary purposes. (" Quart. Microsc. Journ.," vol. iii, p. 87.) Such 

 an instrument, when its axis does not form a very large angle 

 with that of the microscope, may receive its light from the plane 

 mirror, especially if this be so-mounted as to be capable of being 

 turned considerably out of the visual axis ; but when its position 

 is too oblique for the light to be thus supplied to it, recourse 

 must be had to rays either proceeding direct from their source 

 (such as a lamp or a bright cloud), or directed at the requisite 

 angle by a reflector placed in a suitable position. For this latter 

 purpose, a rectangular prism (§ 57), mounted on a separate stand, 

 will be found very convenient. By many observers, a combina- 

 tion of the reflecting and refracting powers of a prism is preferred, 

 which causes the rays to be at once reflected by a plane surface, 

 and concentrated by lenticular surfaces ; so that the prism an- 

 swers the purpose of mirror and condenser at the same time. 

 Such a prism was first constructed by Amici ; and it may be 

 either mounted on a separate base, or attached to some part of 

 the microscope-stand. The mounting adopted by Messrs. Smith 

 and Beck, and shown in Fig. 39, is a very simple and convenient 

 one ; this consists in attaching the frame y^f, 39, 



of the prism to a sliding bar, which 

 works in dovetail grooves on the top of 

 a cap that may be set on the cylindrical 

 fitting beneath the stage ; the slide 

 serves for the regulation of the distance 

 of the prism from the axis of the micro- 

 scope, and consequently of the obliquity 

 of the illumination ; whilst its distance 

 beneath the stage is adjusted by the 

 rack-movement of the cylindrical fit- ^"'''''^ ^"^"^ ''"•'""""'= "'™'"'■'"""■ 

 ting. In this manner, an illuminating pencil of almost any de- 

 gree of obliquity may be readily obtained ; but there is no pro- 

 vision for the correction of its aberrations. Such a provision is 

 afforded by the achromatic prism of Mr. Abraham (§ 57), which 

 may be mounted in the manner just described. And the same 

 object is attained by an arrangement devised by Mr. Grubb, a 

 Dublin optician, of which Dr. Robinson, of Armagh, speaks very 

 highly; the prism having its aberrations corrected for a lamp 

 placed at a given distance in the plane of the stage ; and being 

 mounted in such a manner as to be capable of travelling (like Mr. 

 Sollitt's condenser) through an angular range of as much as 120° 

 (" Quart. Microsc. Journ.," vol. iii, p. 166). In all of these me- 

 thods, the obliquity of the illumination is practically limited by 

 the construction of the stage, and especially by the relation 

 which its thickness bears to the diameter of its lower aperture. 

 The thinner the stage, and the larger its lower aperture, the 

 more oblique will be the rays which may be transmitted through 

 it ; and in admitting an extreme obliquity of illumination, the 



