1906-1907.] Address by the President. 427 



It is, however, often desirable that minute objects, although 

 transparent, should be viewed by means of reflected light by 

 object - glasses of high power. To enable this to be done, 

 a vertical ilhmiinator was invented. This consists of a short 

 tube which screws on to the microscope between the nose- 

 piece of the latter and the object-glass. There is an aperture 

 in the side of the tube for the admission of light, and in the 

 inside of the tube is a small mirror which is controlled by a 

 pin on the outside. The light from the lamp placed in front 

 of the aperture is reflected by the mirror down through the 

 object-glass to the object, and then re-reflected up through 

 the tube of the microscope to the eyepiece. It is a mode of 

 illumination somewhat difficult to manage, and is of use only 

 with wide-angled homogeneous immersion object-glasses. 



Transmitted Light. — The great majority of microscopic 

 objects have to be viewed by transmitted light, — that is, 

 light sent up through the object, not reflected from it, into 

 the object-glass. Of course, all objects viewed by this mode 

 must be transparent. To change the direction of the rays of 

 light coming from the lamp and transmit them to the object, 

 the mirror which is supplied with every microscope is used. 

 The mirror has usually one side 'plane and one side concave. 

 The former side is used when a weak diffused light is em- 

 ployed in examination of the object, and the latter side when 

 a narrow brilliant light is required. A good distance of the 

 lamp from the mirror is about nine inches. There is no great 

 difficulty in working with the plane side of the mirror : we 

 have only to keep in mind that the angle of the reflected 

 rays is equal to that of the incident rays. The concave side 

 of the mirror requires more attention. A concave mirror, in 

 reflecting rays of light, acts in the same way as a convex lens 

 does in refracting them — that is, it brings the rays to a point, 

 renders them parallel, or makes them divergent, according to 

 the position of the source of light. We must, then, so adjust 

 the light and the mirror as to bring the rays reflected from 

 the latter to a focus or point on the object under examination. 

 Speaking generally, when parallel rays fall upon a concave 

 mirror, the reflected rays are brought to a focus at a distance 

 of about half the radius of the mirror. That is, if the radius 

 of the mirror is six inches, the focal point of the reflected 



