362 



KNOWLEDGE & SCIENTIFIC NEWS. 



[February, 1906. 



Conducted by F. Shillington ^Scales, b.a., f.r.m.s. 

 Elementary Photo-micrography. 



{Continued from page 336.) 

 The axis of the microscope should point straight 

 down the camera baseboard towards the light, parallel 

 with the sides; it must be truly horizontal and the disc 

 of light should appear exactly in the centre of the 

 ground-glass screen. Now remove the objective and 

 the ground-glass screen and look straight along the 

 tube, and then adjust the illuminant until it appears in 

 the centre of the limited field of view. Next move the 

 camera up in its slides until there is room for the head 

 to be placed between it and the microscope in order to 

 enable the eye to look down the tube. It is a conveni- 

 ence in this respect if the whole portion of the camera 

 base which carries the microscope, lamp, Ike, rotates 

 on a pivot to one side. 



We have now got the microscope in the place it will 

 occupv in photographing, and we have also got the 

 source of illumination approximately centred, but have 

 still to centre the sub-stage and auxiliary condensers. 

 First centre the sub-stage condenser. This is done in 

 the usual way by using an ordinary eye-piece and a 

 fairly low-power objective, say one inch, closing the 

 iris-diaphragm as much as possible, and racking the 

 body-tube up or the condenser down until the edges of 

 the diaphragm appear in the field, so that the small 

 central disc of light can be centred with the centring 

 screws. Then focus the objective upon some suitable 

 slide placed upon the stage and focus the condenser. 

 If the image of the illuminant does not appear in the 

 centre of the field it must be brought there by moving 

 the illuminant laterally and not the condenser. This is 

 sometimes rather a troublesome job as very small ad- 

 justments are required, and the hands cannot reach the 

 light w-hilst the eye is at the microscope tube, so that 

 assistance from a second person is of service. The 

 auxiliary condenser now alone remains to be centred, 

 and this can only be done by means of a cap of brass 

 or blackened cardboard fitting on its anterior face, and 

 perforated with a very small hole. This small hole is 

 focussed by means of the sub-stage condenser, but it 

 is brought central by moving the auxiliary condenser 

 vertically or horizontally, and again not by re-centring 

 the sub-stage condenser. Care must be taken that the 

 auxiliary condenser is set true and square with the long 

 optic axis, to which we have so carefully adjusted 

 everything. It must then be decided at what distance 

 from the lamp the auxiliary condenser shall be placed, 

 whether to give parallel or convergent light, and if the 

 latter, where the light shall come to a focus. The ex- 

 periments with the white paper will prove very useful 

 here. The blackened disc is, of course, removed after 

 the centring, just as the iris-diaphragm of the sub- 

 stage condenser is re-opened. On examining the 

 ground glass screen, a bright and uniformly illumin- 

 ated disc should be observed in the centre of the screen. 

 Of course, with a one-inch objective the sub-stage con- 

 denser will probably not give a big enough field of 

 light, in tliis case the top lens can be removed. 

 (To be continued.) 



Royal Microscopical Society. 



December 20, at 20, Hanover .Square, Ur. Dukinlield 

 II. Scott, F.K..S., president, in the chair. 'Hie Presi- 

 dent called attention to a donation of slides prepared 

 by .\ndrew Pritchard, about 50 years ago, presented 

 to the Society by Mr. N. I). F. Pearce, which were ex- 

 hibited under microscopes in the room. Mr. Rheinberg 

 called attention to an exhibit consisting of about 20 

 photographs of diatoms, taken by the Zeiss apparatus 

 designed by Dr. .August Kohlcr, of Jena, for photo- 

 micrography with ultra-violet light, having a wave- 

 length of 275 MM (see '■ Knowledge " for June, 1905, 

 p. 138). Tlie photographs were taken with a 1.7 m.m. 

 monochromatic objective of 1.25 N..'\., using light from 

 the cadmium spark, and having a resolving power 

 equivalent to a N.A. of 2.5, with an objective used with 

 ordinary light, were such possible. There were photo- 

 graphs of Surirella gemma, and Amphipletira peUucida, 

 one of the latter, taken with oblique illumination, show- 

 ing the diatom clearly resolved into dots. Photographs 

 of the same diatom taken with a 2 m.m. apochro- 

 matic objective of 1.4 N..\., and light from the mag'- 

 nesium spark (x = 383 mm), and at the same magnifica- 

 tion, i.e., 1800 diameters, were shown for comparision, 

 and the difference was very apparent. Mr. Curties 

 said that this was not the first time A. pcUucida had 

 been resolved into dots, for he remembered such a 

 photograph being made by Mr. Gifford, whilst Dr. 

 Spitta showed the diatom itself at one of the Society's 

 meetings, but the resolution in these cases was less 

 distinct than in the photographs now shown by Mr. 

 Rheinberg. Mr. D. M. S. Watson read a paper on 

 " .\ Fern fructification from the low-er coal measures of 

 Shore, Lancashire," and exhibited a large section of 

 the coyJ under the microscope, also lantern slides to 

 illustrate the paper. In the ensuing discussion, the 

 President, Professor F. W . Oliver, and Mr. E. A. 

 Xewell Arber, took part. 



Quekett Microscopical Club. 



The 426th ordinary meeting was held at 20, Hanover 

 Square, \V., on December i^, the President, Dr. li. J. 

 Spitta, F.R.A.S.. F.R.M.S.,^in the chair. 



Mr. Bryce read a description by Mr. J. Murray, of a 

 new Bdeiloid Rotifer, from Upper Sheringham, Nor- 

 folk, under the name of Callidina vesicularis. It some- 

 what resembles the well-known C. qnadriccrnifera. 



Tlie President then delivered a paper on " Some Ex- 

 periments relating to the Insect Compound Eye." 

 -After some preliminary remarks, he reminded his 

 hearers of the general formation of tlie human eye, 

 laying especial stress on the extremely delicate nature 

 of the retina. Considering the insect compound eye, 

 he said one of the chief differences to^ be noted was that, 

 in the human eye the retina was separated from the 

 cornea by a lens (not to mention two fluid media), 

 while in the insect, the retina was in actual contact 

 with the back of the cornea. He suggested that there 

 was no real lens at all in the eye of most insects, and 

 quoted the last edition of Dr. Packard's " Entomology" 

 in support of his statement. He believed that the 

 cornea might be considered as a series of .sm;dl holes. 

 The lecturer then reminded his audience how the 

 multiple images of the so-called insect-eye were ob- 

 tained in the microscope, and attention was drawn to 

 the fact that, to make the experiment, only the cornea 

 of the eye is used by the microscopist — the soft parts 

 being usually w-ashed away, or otherwise destroyed in 

 the mounting. .After referring to several of the difficul- 

 ties met with by those who accepted the orthodox 



