I860.] ^03 [Frazer. 



A Mirror for Illuminating Opaque Objects for the Projecting Microscope. 

 By Persifor Frazer, Jr. 



(Read before the American Philosophical Society, Feb. 20, 1880.) 



The subject of the present note is an arrangement for representing 

 opaque objects through the gas microscope, especially adapted to Zent- 

 naayei^'s 1| inch objective. It is only claimed to be better than the para- 

 bolic reflector of Smith & Beck, J. Lawrence Smith, Sorby and others, 

 where the working distance of the microscope is comparatively large 

 (i. e., the distance from the objective to the object on the stage is £ 

 inch or more) and for the purposes mentioned. Where the distance is as 

 great as that just mentioned the dispersion of rays from the reflexion atone 

 point, of rays from very different parts of the mirror, is so great that only 

 a few rays from the upper part of the mirror reach the lens at all. It would 

 be different with a lens having a very small working distance, and in this 

 case a parabolic reflector would be preferable. 



The apparatus consists of a brass tube made to slide over the lens, on the 

 lower end of which is fixed a glass plate about 1 mm. in thickness eo 

 attached as to be capable of a sliding motion towards or away from the 

 hinged mirror which is attached to the edge of the metal flange in which 

 the glass plate slides. This simple contrivance permits the glass plate to be 

 brought into close contact with the reflecting mirror no matter at what 

 angle the latter may be placed. 



The mirror is made of nickel-plated German silver neatly mounted on a 

 small hinge. 



The light is admitted from below through a diaphragm after the rays 

 have been rendered parallel by the condenser of the lantern, the aperture of 

 the diaphragm being adapted to the maximum thickness of beam which 

 can be effective for illumination, and which (calling a the aperture of the 

 lens and i the angle of incidence of the beam) = a cos i: or for an aper- 

 ture off" (=0.875") and an incident angle of 62°, 0.411" or roughly 0.4". 



The less the incident angle of course the larger the beam of light will be, 

 and the greater diameter of the diaphragm. The refractive index of the 

 glass employed to make the plate being 1.5, in order that the critical angle 

 41° 48' may not be exceeded in the refracted ray, this angle of incidence 

 or i must not be less than 61° 51' or roughly 62°. 



This minimum value of i determines the area of surface which can be 

 illuminated on the microscope stage, but by altering the angle of the mirror 

 very slightly all parts of the object may be successively projected on the 

 screen. This minimum value is easily obtained from the critical angle of 

 the glass employed, which is 41° 48'. The complement of this, or 48 D 12', 

 is equal to the angle of refraction (or r) when the minimum value of i 

 is attained. 



48° 12' 



