1906-1907.] Address by tJie President. 425 



bears a Herschel condenser, p. The condenser is fixed at any 

 inclination by a milled head working in a slotted piece of brass, 

 Z, fixed to the arm, D, and it can also be used on either side 

 of the chimney, as shown in the illustration at M. This is a 

 lamp which meets every requirement of the microscopist. I 

 have used both the foregoing lamps for many years, and 

 have found them in every respect satisfactory. 



Accessories for Illumination. — There are two modes of using 

 illumination for the microscope — by reflected light and by 

 transmitted light. The former is principally used for viewing 

 opaque objects, and we will consider it first. The microscope 

 being placed in position, the lamp is set on the left of the 

 observer, about eight inches or so from the microscope, and 

 raised on its support till the flame is somewhat above the stage 

 of the microscope. A condensing-lens is placed between the 

 lamp and the microscope : this^, condensing-lens may be on a 

 separate stand or affixed to the stand of the microscope, and it 

 is placed in such a position that the object to be viewed is in 

 its focus — that is, where the light passing through the lens is 

 brought to a point. If the condensing-lens is a bi-convex one, 

 it is immaterial which of its sides is turned towards the lamp : 

 it is otherwise if the condensing-lens is plano-convex, and this 

 is the usual form of such lenses, in which case the curved side 

 must be towards the lamp. The lamp and condensing-lens 

 must now be adjusted till a brilliant light falls upon the 

 object. The edge of the lamp flame should be used. 



In Fig, 1, Plate XLIX., are shown the somewhat relative 

 positions of the different things, in which L represents the 

 lamp flame, c the plano-convex condensing-lens, and the 

 object to be viewed. A very efficient means of obtaining 

 light for the examination of opaque objects is by the use of 

 a parabolic illuminator. This is a small speculum of silvered 

 metal which fits on to the object-glass of the microscope, and 

 reflects the light from the lamp on to the object. For this 

 purpose it is best to have the flame of the lamp on a level 

 with the illuminator ; and if a condensing-lens is interposed, 

 it should be so placed as to render the rays of light parallel. 



As we shall again have occasion to refer to parallel rays, it 

 may not be amiss to mention here how the diverging rays of 

 light from a lamp can be made parallel. We have seen that 

 in order to make rays of light from a lamp converge to a point 



