666 SUMMARY OF CURRENT RESEARCHES RELATING TO 



hole in the stage (and may thus be used for viewing opaque objects), 

 but if one side of the plate is bent down, it leaves a wedge-shaped space 

 between the plate and the under side of the stage, the apex of the 

 wedge being toward the centre, and the base toward the outer edge of 



the stage. If the mirror be placed 

 ^*^' "^ ■ on the same side as the base of 



J this wedge-shaped space, a beam of 



^ 



light can, upon swinging it out to 



a proper obliquity, be sent in upon 



the object, and no light less oblique 



can reach the object from below. 



G^ As the ferrotype plate is perfectly 



A B C D is a section of the stage, pliable, it can be bent to any de- 



E F G is the bent ferrotype plate gired angle, and thus light of any 



forming the so-called V-diaphragm. ^iggired degree of obliquity that 



the stage will admit can be obtained, quite free from any light of 



less obliquity. 



" With wide-angled lenses this has a remarkable effect in in- 

 creasing the sharpness of resolution of fine-lined objects. 



" With dry lenses of lesser angles the arrangement can be utilized 

 to produce very fine dark-field effects, the diaphragm, in this case, 

 being bent down nearly to the limit of the semi-aperture of the lens 

 in use." 



Dr. Blackham, in a previous article,* stated that with this single 

 apparatus and a Tolles i of 1' 12 N. A. (= 95" balsam angle) he had 

 frequently gone through the Moller balsam-mounted Probe-platte, 

 resolving every diatom on it, using light from a common kerosene 

 hand-lamp and concave mirror. 



Value of Swinging Substages. — Considerable difference of opinion 

 exists on this point, and the following is a statement of both sides of 

 the question. The first seven paragraphs have been standing in type 

 for some time, waiting for an opportunity for discussing the matter at 

 one of the meetings of the Society .f 



Mr. Grubb, in introducing his form of swinging substage to the 

 Eoyal Irish Academy in 1853,J very clearly foresaw the advantage it 

 would produce for oblique illumination. He dwelt particularly on 

 " the power of directing the illuminating beam on the object at all 

 angles of incidence . . . , and of registering the position at which 

 peculiar effects are obtained." With Mr. Grubb's Microscope, how- 

 ever, the varying effects of obliquity of the illumination were 

 obtained only by continued readjustment of the reflecting prism ; the 

 observer could not conveniently watch the effect of the changes in 

 obliquity whilst actually making them — as with the Thury-Nachet 

 traversing substage,§ which we regard as a very perfect realization of 

 the advantages to be obtained by gradations of oblique illumination. 

 The principle in this latter is indeed so good, that we hope to see it 



* Amer. Journ. Micr., v. (1880) p. 44. t See infra, p. 713. 



X Proc. K. Irish Acad., v. (1853) pp. 296-7. See this Journal, iii. (1880) 

 p. 1056. 



§ See this Journal, iii. (1880) p. 1060. 



