238 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the hemisphere, is intended to nullify the influence which the 

 spherical surface of the hemisphere exerts on the paths of the rays. 

 The objective 0' can also be provided with a correction lens, when 

 using the combination for the vision of very distant objects by re- 

 flexion on the plane surface of the hemisphere ; this property is use- 

 ful for the adjustment of the hemisphere. For small fragments the 

 combination used consists of an objective 0, composed of an achro- 

 matic lens of about 40 mm. focal length, with a correction lens and a 

 special ocular. This ocular fits with gentle friction into the tube u 

 of the instrument, and bears at its anterior end a network in the focus 

 of the objective ; the anterior lens (divergent) giving, in combination 

 with the objective, an enlarged image (4-5 diameters) of the object 

 placed on the hemisphere. This image is formed in the plane of an 

 iris diaphragm i, which, for more convenience, can be laterally displaced 

 by the aid of the screws r. The image is viewed by the'loup /. When 

 the loup I is replaced by another of a focus giving vision of the net, 

 this optical combination, which is a real Microscope, is converted into 

 a telescope directed on infinity, and by it the phenomenon of total 

 reflexion can be observed. A nicol prism N fitted w T ith a graduated 

 circle can be easily adapted to either of the two combinations without 

 derangiug the observations. Perfect centring of the objective is 

 obtained by the action of three screws not shown in the figure, and 

 that of the hemisphere by the three screws 1, 2, 3. The makers supply 

 full instructions for the use of the instrument. 



Beck's New Illuminator for High-power Dark-ground Illumination.* 

 This apparatus permits of dark -ground illumination, with object-glasses 



Fig. 48. 



as high as a T V in. oil-immersion. The principle is that of a reflecting 

 paraboloid, specially designed to obviate the difficulty arising from the 

 immersion fluid running down the side of the paraboloid and the 

 consequent impossibility of adjusting the focus. The new illuminator 

 is made of two parts, which may be more or less separated, and tin's 

 enables the light to be focused, according to the thickness of the slip on 

 which the object is mounted, and the oil is kept away from the reflecting 

 surface. The lower portion consists of a reflecting paraboloid B 

 (fig. 48), reflecting parallel light to a focus at C, with a concave upper 

 surface. The upper portion of the apparatus is in the form of a lens A, 

 with focus at C, the upper surface of which is placed in immersion 

 contact with the under surface of the slip ; the curved side is concentric 



* R. and J. Beck, London, Special Catalogue, 1908. 



