I QO THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE 



being moved laterally out of the axis in an arc which has the object on 

 the stage for its centre. 



The sole purpose of this is to secure oblique illumination, which 

 practically, at the time the swinging sub-stage was devised, meant 

 obtaining a more oblique pencil than the condensers then provided 

 could command ; and since this also meant sending into the object a 

 small portion of a cone of light in one azimuth, many tacitly assumed 

 that this alone was taken to be ' oblique illumination.' But whatever 

 sends oblique light through an object into the objective is an oblique 

 illuminator. Two condensers may have numerical apertures of T4 

 and 1'5 respectively; a stop behind the back lens in each has a 

 narrow sector cut out, representing the conditions of the so-called 

 ' oblique illuminators ; ' by the former we get an oil angle of 134 10', 

 by the latter a similar angle of 161 23'. These sectors of the cone 

 of light of 67 5' and 80 41' respectively are in every sense 

 ' oblique illuminators,' and the one more oblique than the other. 



Whether or not it is needful or best to use such a sector is 

 scarcely an open question ; it is manifest that by taking the stop 

 with its sector away from each condenser and sending in the complete 

 cone of light formed by the condenser, we are still using oblique 

 illuminators, but the obliquity is in all azimuths. 



There can be no doubt that a large aperture in a condenser 

 provides the microscopist with far greater wealth of resource than an 

 oblique illuminator in one azimuth can ever give him. A condenser 

 with an oil angle of 161 23' is much more valuable than even the 

 semi-angle obtained by a mere section of a luminous cone. The 

 power to utilise the entire cone is a gain of the highest order. 



It will be manifest to all that we want concentration as well as 

 obliquity. 



Ordinary concentration depends upon the power of the condenser. 

 If it is required to concentrate the light from the edge of the flame 

 of a paraffin lamp upon an Amphipleura pellucida, the condenser 

 must be at least a th inch or 1th inch in power, which will give an 

 image of the flame nearly the same size as the object. The amount 

 of light which is concentrated upon that object will of course depend 

 upon the aperture of the condenser. An oblique cone of great in- 

 tensity is here what is needed ; the illuminating cone should be 

 equal and conjugate to that which exists between the object and the 

 objective. 



Now it is certain that this condition cannot be met by an ' oblique 

 illuminator ' of the kind commonly undersood by that name ; to get 

 immersion contact, which is of course a sine qua non, we must employ 

 a hemispherical button or one greater than a hemisphere placed 

 in immersion contact with the under surface of the slide. This may 

 be illuminated by a beam from a dry combination, made oblique by 

 the sub-stage being swung out of the axis. Granted that the angle 

 is attained which can be got with a condenser of great aperture, we 

 manifestly obtain only a portion, and an attenuated and small por- 

 tion, of the light given in every, or at will any, azimuth by the con- 

 denser. 



Theoretically perfect illumination of an objective, for example, 



