1882.] 



MICROSCOPICAL JOURNAL. 



40 



sion at one of the late sessions of the 

 Microscopical Society of London. 



The speaker has made some im- 

 provements upon the model of R. & 

 J. Beck, and added an adjustable dia- 

 phragm, by means of which the in- 

 strument is made more useful and 

 gives better results. Moreover, the 

 reduced dimensions of the apparatus 

 thus modified permits of its use — with 

 Zeiss homogeneous objectives — on 

 microscopes of the smallest model. 

 With a very simple and cheap acces- 

 sory one may thus resolve the most dif- 

 ficult tests, A.pullucida and the 19th 

 band of Nobert's plate, more perfect- 

 ly than with immersion condenser. 

 Woodward's prism, swinging sub- 

 stage, etc., which are only applicable 

 to large microscopes. 



By means of an apparatus con- 

 structed by M. De Simpelaere, of 

 Brussels, and a j^-inch objective of 

 Zeiss, several tests were successfully 

 shown, — Surirella gemma, Navicula 

 rhomboides, Amphipleura pellucida, 

 and the scales of Poduraplumbea, were 

 resolved with extraordinary clearness. 

 Under a magnification of 3200 diame- 

 ters, the striae of A.pelliuida appeared 

 manifestly separated into pearls or 

 dots. The appearance of Pleurosig- 

 ma atigulaturn under this illumination 

 is very unexpected. The markings 

 resemble the alveolar or hexagonal 

 structure of certain species of Trice- 

 ratium, Coscinodiscus, etc. This spe- 

 cial illumination affords a new confir- 

 mation of the opinion held by some 

 microscopists regarding these mark- 

 ings, and which M. Rutot reproduced 

 at the stance of October 30th, 1880, 

 a propos of the microphotographs of 

 this Pleurosigtna prepared by Dr. 

 Giinther, of Berlin. 



The appearance of solidity, of relief, 

 afforded by objects thus illuminated, 

 the preservation of their natural col- 

 ors, the clearness presented by the 

 smallest details of their surface, have 

 led many observers to admit that the 

 effects are due to an illumination very 

 similar to that which reflected light 

 produces with opaque objects. But 



Mr. Stephenson seems to have demon- 

 strated that the rays condensed upon 

 the object, at least when it is transpa- 

 rent, as are the silicious valves of the 

 diatoms, are not reflected from the su- 

 perior surface, and that they traverse it, 

 on the contrary, without illuminating 

 it. Only one portion of the rays, of 

 which the obliquity exceeds the criti- 

 cal angle (41°), suffer total reflection 

 from the inferior surface of the ob- 

 ject, and instead of passing into the 

 air, illuminate it by making it self- 

 luminous. Thus is explained why 

 the apparatus only produces this par- 

 ticular illumination with objectives 

 having angle of aperture greater than 

 the maximum of dry-objectives, more 

 than 180°, and on preparations mount- 

 ed dry having the object adhering to 

 the cover. 



Dr. Ermengem, moreover, called 

 the attention of the members to the 

 advantage of this method of illumina- 

 tion in the study of certain histologi- 

 cal elements. There are many ana- 

 tomical preparations which can be 

 temporarily mounted dry on the cov- 

 er-glass. Human blood cells pxe- 

 sented an extraordinary appearance, 

 in their bright-red color, their very ap- 

 preciable relief, and the clearness with 

 which the slightest inequalities of 

 their surfaces are visible. 



The Limiting Diaphragm, or 

 Aperture Shutter.* 



For many years past we have heard 

 declamations against the practical 

 value of wide aperture objectives, 

 principally, however, upon the ground 

 that they do not possess /(f^^/ra//*?/?, 

 though all seem agreed that they do 

 define much better than those of 

 small or medium angle. Even Dr. 

 Carpenter, the champion of medium 

 angles, in the last edition of his work, 

 p. 732, seems at last to prefer wide 

 apertures, for in the only instance in 

 which he has expressed an opinion 

 upon the structure of insect scales 

 from his own study he writes : " The 



* From The Northern Microscopist. 



