ZOOLOGY AXn BOTANY, MICROSCOPY/ ETC, 229 



In the present series of experiments the depth of the illuminated 

 part of the solutions was kept smaller than before and restricted to 

 10-12 ytt, so that in an image-breadth of 27 /x, a cross-section of about 

 ;-500/A^ was illuminated. The net in the ocular of the observation micro- 

 scope contained eighteen squares, and coiTesponded to 18 X (9 ^a)^ of an 

 upper plane on the object. The net therefore covered an illuminated 

 volume of 11 x 18 x 81 ju,^ (roughly 16000 /a^); and a single square 

 would cover about 81)0 /u.^. The task of counting reduces itself, in the 

 case of solutions rich in particles, to estimation of those on one square ; 

 in the case of weaker solutions, to those on the whole net ; while, in 

 the case of fairly empty solutions, those in the entire field (about five 

 times the size of the net) must be counted. In the observations special 

 attention was paid to the number of particles, their movement, their 

 colour, and their brightness ; also to the determination of the relative 

 number of the particles of differing colour and brightness and to the 

 age of the solution under examination. The author adopts Siedentoff 

 and Zsigmondy's terms Salmicrons and Amicrons ; the former means 

 particles ultramicroscopically perceptible ; the latter, those beyond the 

 reach of the ultramicroscope. The author gives in seven tables the 

 results of his efforts. 



Iris of Optical Systems.* — A. E. Conrady explains a certain pecu- 

 liarity of microscopic images referred to by J. E,heinberg,t viz. that when 

 two objects of exactly the same size were placed at different distances 

 from the object-glass (but within reach of its depth of focus), the more 

 distant one was not always depicted as the smaller, but did sometimes 

 actually yield the larger image. As a rule, there is in every optical 

 system one aperture which limits the diameter of the cone of rays 

 passing from any point in the object to the conjugate point in the 

 image. This aperture Abbe called the iris of the system. By means of 

 diagrams the author shows that the images are reversed according as the 

 aperture is placed in front of the object-glass or behind its upper focal 

 plane. There is an important intermediate case, viz. when the iris is 

 placed exactly in the upper focal plane, the consequence being that 

 only those rays can pass which had been parallel to the optical axis 

 before entering the object-glass. The image so produced is definite 

 and unchangeable. Professor Abl)e was the first to point out the 

 value of this arrangement for measuring instruments, and introduced 

 the term " telecentric " for object-glasses with the limiting stop in this 

 particular position. 



On Stereoscopic Effect and a Suggested Improvement in Binocular 

 Microscopes. J — Julius Rheinberg, after pointing out that a proper 

 understanding of the subject implies adequate recognition of the fact 

 that stereoscopic vision with the Microscope means viewing objects in 

 three dimensions, of which only a single plane is in perfectly true focus 

 at any one time, passes on to show that points in all other planes are 

 represented in the view plane by diffusion disks, which may vary not 

 only in size, but in shape and position. It is shown that their size 



* Joum. Quekett Micr. Club, ix. (1906) pp. 440-2 (3 figs.). 



t Tom. cit., p. 375. % Tom. cit., pp. .371-96 (9 figs.). 



