Jll.v, 1012. 



kxo\vli:dge. 



275 



No light is necessary ; on the other hand, a dark room is 

 not needed, for the whole operation can be carried out on 

 the laboratory table, either in the daylight or by artificial 

 light. 



Tissues prep.ired in this way are stained in black and white, 

 thus providing a good contrast. 



The accompanying photograph gives a good idea of the 

 result obtained by this method. 



C. E. Jenkins, 



Pliysiological Laboratory. University 



College. Cardiff. 



A DOUBLE 

 PIECE. — This 

 introduced by 



DEMONSTRATING EVE- 

 eyepiece, which has been 



Mr. E. Leitz, enables two 

 observers to view jointly an object under the 

 microscope. It slips into the draw-tube of 

 the microscope like an ordinary eyepiece. 

 The field of view is common to both eyepieces 

 and contains a pointer which cither observer 

 can direct upon any feature to which he 

 wishes to draw attention. 



The arrangement of the device is shown 

 in Figure 304. 



I and II are two prisms in contact and 

 mounted above the diaphragm between the 

 field lens and the eye lens of the eyepiece. 

 The prism I has an isosceles cross section 

 and its angles are 35°, 35°. and 110° res- 

 pectively. The prism II is rectangular, and 

 its angles are 35°, 55°, and 90°. The prisms 

 are placed with those faces in contact which 

 subtend the angles of 90° and 110° in such a 

 manner as to leave between them a very thin 

 film of air. This film is inclined at an angle 

 of 30° to the axis of the eyepiece and partially reflects the 

 emerging pencil of rays ; about two-thirds of the rays pass 

 through the prisms, and one third is reflected. 



The image formed along the axis of the microscope is accord- 

 ingly brighter than that produced by partial reflection. The 

 centre line of the reflected pencil is inclined at an angle of 70° 

 to the axis of the microscope. 

 Ill is the prism, the lower 

 surface of which reflects the 

 pencil upwards at a con- 

 venient angle for observa- 

 tion. In order that the two 

 observers may not be in 

 each other's way the branch 

 tube is fitted with a system 

 of lenses which resembles 

 a terrestrial eyepiece. The 

 image as seen in the side 

 tube is reversed with res- 

 pect to that which appears 

 in the axial eyepiece ; but 

 this would hardly affect the 

 observer, especially since 



the oblique attachment of the side eyepiece already introduces 

 unusual conditions of working. 



As a matter of fact, the more expedient course is to adjust 

 and focus the object through the principal eyepiece, as the 

 image seen through it is brighter and easier to focus. The 

 adjustment for one eyepiece furnishes also a clearly-defined 

 image in the subsidiary eyepiece, provided the eyes of both 

 observers can accommodate in a similar manner. The objec- 

 tive in conjunction with the field lens below the double prisms 

 of the two eyepieces forms an image in the plane of the dia- 

 phragm below the double prism. This image and the pointer, 

 being both in the plane of the diaphragm, are seen simul- 

 taneously in the principal and the subsidiary eyepieces. 



The pointer can be moved backwards and forwards and 

 turns on a pivot, so that its extreme end can be set at any 

 point in the field. The Double Demonstrating Eyepiece is 

 made in two powers, one having a magnification of four 

 diameters, and the other of six diameters. In both cases the 



Figure 304. 



rhe Double-demonstrating 

 Eyepiece in position. 



Figure 305. 

 Details of the Double-demonstrating Eyepiece. 



resulting images are sharp, colourless, and free from distor- 

 tion. The fact that the image seen in the subsidiary eyepiece 

 is fainter than the other is no serious drawback, as the eye- 

 pieces are solely intended for demonstrating purposes, and the 

 demonstrator's acquaintance with the object will generally 

 enable him to see every detail clearly under these less perfect 

 conditions. When diffused daylight does not suffice to bring 

 out fine details distinctly in the darker portions of the field, it 

 will be necessary to use one of the artificial illuminators which 

 are generally to be found in laboratories such as electric glow- 

 lamps, arc lamps, Welsbach or acetylene lamps. With high 

 power objectives it is generally advisable to 

 set the draw tube of the microscope about 

 one centimetre shorter than its standard 

 length. 



The double demonstrating eyepiece is also 

 well adapted for the instantaneous photo- 

 graphy of living bacteria, and other moving 

 organisms illuminated by means of a dark 

 ground condenser. It enables one to watch 

 the object through the side eyepiece, and to 

 defer the exposure until a favourable moment 

 presents itself. 



THE ROYAL MICROSCOPICAL 

 SOCIETY — PRESENTATION TO MR. 

 FREDERICK A. PARSONS.— A very pleasing 

 little ceremony took place at the meeting of 

 the Royal Microscopical Society on June 19th. 

 The President, Mr. H. G. Plimmer, F.R.S., 

 presented to Mr. Frederick A. Parsons (on his 

 retirement from the office of Assistant Secre- 

 tary after sixteen years service! with an 

 illuminated address and a cheque, on behalf 

 of the Fellows of the Society. Mr. Plinnner 

 further mentioned that the Council had reinstated Mr. 

 Parsons as a Fellow of the Society, and had passed a 

 resolution that all annual contributions in the future should 

 be remitted. This announcement was received with great 

 applause, and Mr. Parsons was evidently much pleased 

 with the recognition that had been made of his efforts. 



OUEKETT MICRO- 

 SCOPICAL CLUB.— 

 May 2Sth. — Mr. E. M. 

 Nelson, F.R.M.S., wrote 

 that he had examined a 

 mount, supplied by Mr. 

 H. F. Angus, of Mr. 

 Siddall's diatoms, showing 

 the so-called pseudopodia 

 (see " Knowledge," May, 

 1912, page 193). Using 

 a A-in. Leitz apochromat, 

 structure like a spiral fila- 

 ment in a tube was ob- 

 served. This was in a 

 C o s c i n o d i s c it s. I n a 

 resembled aminutely - jointed 



Biddiilphia the structure 

 antenna. 



Mr. R. T. Lewis. F.R.M.S., read "A note on Solpuga 

 (fcrox?)" This genus is included in the fifth order of the 

 Arachnida. About fifty species are known, all African. In 

 length the adults measure from one to two inches and vary in 

 colour from reddish-brown to dull grey. They are covered 

 with hairs of several distinct kinds. They are armed with two 

 pairs of enormously developed chelicerac placed near together 

 side by side and opening vertically. Two large simple eyes 

 are present. The cephalothorax is formed of six segments, 

 the first three fused. Spinning organs are absent. No poison- 

 sac or duct has been found. On the dorsal surface near the 

 extremity of each of the chelicerac in the male, there is a 

 curious organ, the flagellum, the function of which is unknown. 

 Of the five pairs of lateral appendages the first are the 

 pedipalpi, six-jointed. The first, second and third pairs of 

 legs are also six-jointed. The fourth pair is very 



