126 



APPENDAGES TO THE MICROSCOPE. 



Messrs. Powell and Lealand a piece of plane glass is placed at an 

 angle of 45° across a tube placed like an Adapter between the Objec- 

 tive and the body of the Microscope ; and whilst a pencil of light, 

 entering at the side aperture and striking against this inclined 

 surface, is reflected by it downwards through the Objective on 

 to the object, the rays proceeding upwards from the object pass 

 upwards (with some loss by reflexion) through the plane glass into 

 the body of the Microscope. For this fixed plate of glass, Mr. K.. 

 Beck substituted a disk of thin glass attached to a milled-head 

 (Fig. 74, b), by the rotation of which its angle may be exactly 

 adjusted ; and this is introduced by a slot (shown at e, Fig. 74, a) 

 into the interior of an Adapter that is interposed between the 

 Objective (c, cl) and the nose (c) of the Microscope. The light which 

 enters at the lateral aperture (a, a), falling upon the oblique surface 

 of the disk (c, b), is reflected downwards, and is concentrated by the 

 lenses of the Objective upon the object beneath. There is this 

 advantage in the method of Mr. Beck over that of Messrs. Powell 

 and Lealand, that not only does the former give a power of adjust- 

 ment which it is very impoi'tant the Reflector should possess, but 

 also that the natural surface of the thin-glass disk reflects a much 

 larger proportion of the luminous rays impinging a upon it, than does 

 any artificially polished plane. In using this Illuminator, the Lamp 

 should be placed at a distance of about 8 inches from the aperture ; 

 and when the proper adj ustments have been made, the image of the 



Fig. 74 



Beck's Vertical Illuminator. 



