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XIII. — Illuminating Apparatus for the Microscope. 

 By J. W. Gordon. 



(Read June 17, 1908.) 



The illuminating apparatus which is this evening exhibited has 

 been designed as the result of experience gained in working with 

 very high powers. But it is believed that it will be found to 

 comprise several material improvements upon present forms of 

 apparatus even for use with ordinary magnifying powers. 



For successful illumination of the stage of a Microscope it is 

 necessary that the operator should have control over (1) the 

 brilliancy of the light; (2) the form of the luminous disk which 

 constitutes the source of light ; and (3) the angle under which the 

 light is incident upon the object. It is further important that the 

 light source, when in focus, should be a featureless disk, and that 

 all the adjustments relating to the points above enumerated should 

 be susceptible of being independently made. 



These points may best be illustrated by taking notice of the 

 defects exhibited by the various sources of illumination in 

 common use. 



To take, first of all, the ordinary paraffin lamp. The great 

 defect of this light source is that it is too feeble for use with very 

 high magnifying powers. With ordinary magnifications, however, 

 its brilliancy is abundantly sufficient. But here its shape is faulty. 

 The side of the flame is unsuitable for use, because its luminosity 

 and colour vary in different parts of its area. This defect may, 

 indeed, be made good by placing a perforated diaphragm in front 

 of the flame, and using only a selected part as the effective source 

 of light. In that case, however, if a uniform source of light is to 

 be obtained, it is necessary to limit the exposed surface to such a 

 small area that an equal breadth of light-source can be obtained 

 by placing the flame end-on to the Microscope — and this, in fact, 

 is the plan adopted in practice by all experienced microscopists. 

 It has the additional advantage of presenting the flame with its 

 long axis in line with the optical axis of the instrument, with the 

 result that the light is concentrated, since to some extent the 

 remoter parts of the flame shine through the nearer parts, which are 

 not entirely opaque to the flame-light. 



For use with high powers a source of light having an elongated 

 form is, however, very unsuitable, for a reason which will be easily 



