January, 1913. 



KNOWLEDGE. 



23 



black background (right) the cast shadow is apparently absent 



though really present. 



It may be here noted that general experience among photo- 

 graphers shows that with a dark background we require a 



little longer exposure than with 



a light background, all other things 



remaining the same. With certain 



types of subjects it is desirable to 



employ a light background in 



conjunction with a side lighting, 



but without any cast shadow such 



as shown in my last note. 



This can be easily accomplished 

 in those cases where the object 

 can be affixed to an ordinary 



clean, clear glass micro-slip. This 

 is now held by the spring clips in 

 front of the cut out part of the 

 holder, and then a suitable white 

 ground (e.g., postcard) put in a 

 position and angle where it is 

 well and evenly lighted, and a 

 few inches away from the object. 



iJtfJS 22 , we , see this arrangement showing a postcard 

 background and glass-supported tiny shell and in Figure 21 

 we have the result showing the object enlarged 

 about four diameters with a side lighting, 

 light ground and no cast shadows. 



In Figure 23 we have a (practically) white 

 shell supported on a glass slip in a side lighting 

 against a black ground. In this case the 

 ground was a bit of black velvet pasted to a 

 piece of cardboard. It is far enough away to 

 be quite out of focus and shows no texture. 



Next conies the question of a reflector, which 

 in some cases is of special value. In Figure 24 

 we see an object stuck to a glass slip, and a 

 bit of white card close behind it. To our 

 right we see a postcard used as a diffusing 

 and reflecting screen throwing light on to the 

 shadow side of the object. (The postcard is 

 affixed to the base board with a couple of 

 drawing pins.) 



In Figures 25 and 26 we see the effect of the 

 non-use and use of such a card reflector in the 

 case of a light shell showing some curious black markings 

 In the one case we get a dark shaded side and a cast shadow 

 on the background. In the other 

 case we nearly get rid of the cast • 



shadow and also see a great deal 

 more detail on the shadow side 

 of the object as well as seeing 

 more of the inside of the shell 

 mouth. 



These two examples, Figures 

 25 and 26, are precisely similar 

 as regards lighting, exposure, 

 and so on, and in all other 

 respects, except the matter of the 

 reflector. 



One word of warning : do not 

 use a glass mirror, or even a 

 glossy card, or cross lighting and 

 other ugly effects are likely to 

 arise. 



For all work of this kind it 

 is preferable to employ bright 

 sky light rather than direct sun- 

 light. If direct sunlight falls on 

 the window this should be cov- 

 ered with a piece of thin fine white muslin. 



F. C. Lambert, M.A., F.R P S 



1Q R UE x, ET p MICROSCOPICAL CLUB.-November 26th, 

 \ r Resident, Professor A. Dendy, D.Sc, FRS in 



the chair. ' 



The President made some remarks on new species of 



IGURE 20. 



Figure 22 



Holothunans, with special reference to a form of Chiridota 

 found in Port Phillip Bay, Victoria, and described by E C 

 Joshua as Taenwgyrus allani. It was not, as was at first 

 thought, identical with Chiridota dunedinensis Parker. 



Mr. E. M.^Nelson, F.R.M.S., read 

 a paper "On microscope con- 

 struction and the side-screw fine- 

 adjustment." He pointed out 

 that the modern method of placing 

 the coarse-adjustment slide and 

 the body upon the fine-adjust- 

 ment, and the side-pinion fine- 

 adjustment (now so much in 

 vogue), were both invented by 

 Powell, in 1841. The disadvan- 

 tages of modern horizontal fine- 

 adjustments were mentioned, and 

 an improvement suggested by the 

 author, preventing injury to the 

 delicate moving parts, was des- 

 cribed. The same author also 

 made some remarks on " a new 

 low-power condenser," and re- 

 ferred to the impossibility of obtaining an evenly lighted 

 held, under critical illumination with any power lower than a 

 ^^^^^ two-thirds. Substage condensers suitable for 

 low powers are all of too short a focus. A 

 sufficiently long-focus condenser cannot be used 

 because there is not room to focus it, even on 

 a Powell and Leland No. 1 stand. The author 

 had surmounted the difficulty by designing a 

 condenser on the telephoto principle. This 

 was exhibited at the meeting by Mr. Baker. 

 It had an equivalent focal length of four 

 inches, but required only one inch of working 

 distance. 



ROYAL MICROSCOPICAL SOCIETY — 



November 20th, 1912. H. G. Plimmer, Esq., 



t.R.S., President, in the chair. Messrs. E. 



Heron-Allen and A. Earland read a paper on 



the Distribution of Saccammina sphaerica 



M. Sars and Psammosphoera fusca Schulze 



in the North Sea; particularly with reference to 



the suggested identity of the two species 



a * u- -j These Foraminifera, belonging to the familv 



Astrorhizidae, and originally described as from the North Sea 



but occurring also in all the great oceans, have been the 



subject of considerable contro- 

 versy. Dr. Ludwig Rhumbler 

 asserts that Psammosphoera is 

 only an immature stage of 

 Saccammina. As a result of 

 the examination of about one 

 hundred and fifty dredgings 

 made in the North Sea, the 

 authors have no hesitation in 

 affirming that the life-history of 

 Saccammina, as recorded by 

 Rhumbler, is a composite sketch 

 involving three separate and 

 generally recognised specific 

 organisms : — 



Stages I— III represent the life 

 history of Crithionina mamilla 

 A. Goes. 



Stage IV is Psammosphoera 

 fusca Schulze, an extremely 

 variable species, which occurs 

 both free and sessile, but is in 

 , , , all its stages normally recognis- 



able by the absence of a general aperture. 



Stages V-VII represent the complete life-cycle of Saccavi- 



mma sphaerica Sars, so far as it is a shell-bearing organism 



An abstract of a paper by the Rev. Hildenc Friend on 



British Henleas was read. The Henleas are microscopic 



annelids belonging to the family of Euchytraeids. The genus 



Figure 21. 



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