114 



KNOWI.I.DGI. 



Makcii, 1912. 



I'lGURK 117. 



coiitrivaiicr for this note, 



which is shown in I'ig- 



nre 120. At A and 15 wc 



have two ordinary photo- 

 graphic lenses, A and H, 



both of fairly short focal 



lent;th, vis., four and 



three inches. It was pro- 

 posed to combine the 



two in tandem fashion 



by means of a brown 



paper tube. D. but as 



lens B was somewhat 



sm.ilier than lens A. lens 



B had to have its dia- 

 meter brought up to 



match that of A by 



means of an extra bit of 



tube, C. These card 



tubes are easily made 



of brown paper strips 



and office paste, using 



the lens tube as a roller. 



but remembering that the 



paste-wetted paper will 



contract a trifle, the lens tube should be covered with two 

 laps of thick, smooth writing paper. Without this you may 

 find your tube too tight when it is dry. Talcing the diaphragm 

 distance as the nominal "separation," we can apply the usual 

 rule for finding the com- 

 bined focal length of the 

 four and three inch com- 

 ponents. First multiply 

 four by three, getting 

 twelve. Now add four 

 to three, getting seven, 

 and substracting the dia- 

 phragm distance or 

 "separation." say one 

 inch, we get six. Finall> . 

 dividing twelve by six %m 

 get two inches as the focal 

 length of the whole sys- FkH'KI 



tem. As a matter of fact. 



this estimate was shown to be practically right, but the small- 

 ness of the stops of one of the lenses gave an inconveniently 

 restricted field. However, the idea is worth mentioning. 



Finally, Figure 121 shows this combination in use. Note the 

 card support E, Figure 120, to take the front weight of the tube. 

 We here get a side view of the object holder which, when 

 combined with the front view of Figure 115, will enable this 

 item to be easily made. The exposing black card S is shown, 

 and also the reflector with its dowel-peg (cf. Figure 119) 

 inserted in the baseboard. F. C. Lambert,"M.A., F.R.P.S. 



l-"lGlui-: US. 



KOYAL MICK(J- 

 SCOHICAL SOCIFTY. 

 February 21st. 1<)12.— 

 H. (i. IMimmer, l-;s(|., 

 I'.K.S., president, in the 

 chair. Mr. E. J. Spitta, 

 with the help of the 

 projection Lantern, de- 

 monstrated the principles 

 which should influence 

 the photographer in the 

 preparation of negatives 

 from which coloured 

 lantern slides were sub- 

 sequently to be made. 

 Commencing with photo- 

 graphs of the spectrum 

 taken upon the various 

 sensitised emulsions at 

 present on the market, 

 he pointed out the ne- 

 cessity for the use of 

 panchromatic plates. He 

 showed a number of 

 slides, macroscopical as 

 well as microscopical, by means of which he contrasted the 

 effects produced by colouring lantern slides made from 

 ordinary plates with those prepared from panchromatic 

 plates, and showed the wide possibilities which were opened 

 up to the skilful artist. 

 Mr. Rousselet com- 

 municated the " Fourth 

 List of New Kotifera 

 since 1889" ii.e.. the 

 date when Hudson and 

 Gosse's Monograph of 

 the Kotifera was com- 

 pleted by the issue of 

 the supplement. recording 

 altogether four hundrtd 

 species at that tiniei. 

 The author explained 

 ]](l that his three preceding 



lists, published in 1S9J. 

 1897, and 1902, contained three hundred and ninety-three 

 new species, and the fourth list now submitted two hundred 

 and fourteen names, a total of six hundred and seven new 

 species since 1889. After making allowances for synonyms 

 .and insufficiently recorded species, Mr. Rousselet estimated 

 the present Rotiferous population of the world comprised 

 eight hundred and fifty-seven species. .-V slide of Xaiiciilti 

 socialis. presented by Professor T. Chalkley Palmer, 

 was exhibited by the Society. This is a fresh water 

 diatom, discovered by the donor in 1905 near Media, Pa. 



FlGURli 121, 



