170 THE MICROSCOPE. 



Professor Smith, of Kenyon College, America, invented 

 a condenser for the illumination of opaque objects under 

 high powers. It has been felt that it would be of im- 

 mense advantage to the physiologist if he could illuminate 

 small bodies, such as blood-globules, and view them as 

 opaque objects with the Jth or -j^th ; hitherto there have 

 been great difficulties in the way of accomplishing this, 

 but Professor Smith has hit upon a contrivance which 

 promises to be useful. 



In this instrument a pencil of light is admitted above 

 the objective and thrown down through it on to the object, 

 by means of a small silver mirror placed on one side, and 

 cutting off a portion of the aperture. Powell and Lealand 

 devised what they considered to be an improvement, and 

 substituted for the small silver mirror, to which Professor 

 Smith gave a preference, a flat piece of glass placed at an 

 angle of 45° across the tube, interposed like an adapter 

 between the objective and the microscope body. A pencil 

 of light entering by a side aperture striking against this 

 flat glass is partly reflected down through the objective 

 and on to the object, the magnified image of which is 

 viewed through the glass. If the flat glass be ground so 

 as to have parallel surfaces, no great amount of error can 

 be detected. 



Smith and Beck place a disc of thin covering glass, at 

 an angle of about 45°, in the optic axis of the microscope 

 body, close behind the setting of the object-glass in a special 

 adapter, leaving a suitable aperture for admitting light from 

 a lamp, rays from which are reflected downwards upon the 

 object. The object-glass is thus made its own achromatic 

 condenser. 



But the idea of employing the object-glass as its own 

 condenser was suggested by Mr. Hemtt five years ago, 

 and then Mr. Wenham was induced to give the plan a 

 trial. A concave speculum was fitted at an angle into the 

 body of the microscope, having a central hole sufficiently 

 large to admit the full pencil from the objective, through 

 the back of which the rays from a lamp (the light from 

 which was admitted tlirough a hole in the side of the 

 body) were reflected downwards. The object was strongly 

 illuminated, but there was so much glare from the internal 



