556 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Figs. 101 and 102 (where in Fig. 101 A is a vertical view, and in 

 Fig. 102 B a sectional view, and C the prism three-fourths full size). 

 He states that this accessory is easily placed in position in the well- 

 hole of the " Acme " stand, and that doubtless with slight modifica- 

 tions this system of mounting the prism may be applicable to other 

 Microscopes. Provision is made for centering in a lateral direction 



Fig. 101. 



Fig. 102 



by means of the milled head at A. The prism can also be revolved 

 by the milled head shown below so as to use either face, the faces 

 being cut at different angles. Eegarding the angles of the prism- 

 faces, he thinks 98°, 41°, and 41°, as suggested by Dr. Woodward, are 

 good ; but that 93°, 47°, and 40°, which he has himself adopted, are 

 especially adapted for the general run of modern wide-apertured 

 objectives. He gives detailed instructions for the use of the apparatus 

 and thinks that it " bids fair to come into general use." 



Prisms versus the Hemispherical Lens as Illuminators. — In 



various catalogues issued by American opticians, references are also 

 made to sundry forms of mounting for the " Woodward " prism. It 

 appears to us that much ingenuity is being wasted in such efforts, for 

 whatever may be the angles of the prism-faces, the hemispherical lens 

 must necessarily entirely supersede it, having in fact an infinite number 

 of facets through which normal light may reach the common centre. 

 There may of course be cases where a small beam of parallel rays 



