NOTES AND MEMORANDA. 211 



known to him or are received by him from correspondents. If 

 serious students of our pond-fauna and flora will avail them- 

 selves of Mr. Bolton's services, and not only purchase from him 

 examples of the specimens he has on hand, but will also send to 

 him supplies of such rarities as they may Jincl,for the jpurpose of 

 distribution among his correspondents and customers, we shall have 

 started among us an agency which will be of immense service 

 not only to the individual student but also (and perhaps chiefly) 

 to the teacher who requires to be able to obtain supplies of 

 given microscopic organisms for his practical classes and to feel 

 with absolute certainty that the specimens needed will be forth- 

 coming on the appointed day. Mr. Bolton can, at present, be 

 depended on for certain forms ; after a little time he will be able, 

 no doubt, to enlarge his list. E. Ray Lankester. 



Zeiss' Objectives. — We wish to draw attention to some objec- 

 tives made by Carl Zeiss of Jena. We have carefully examined 

 his D, E, and E, and find that they are remarkably fine glasses 

 for his tological work. 



D, -^th, equal to about HartnacFs No. 7 (2 guineas), and 

 E, ^th, equal to about Hartnack's No. 8 (3 guineas), are both 

 glasses of great excellence as regards light, power of definition 

 and penetration. The D and E that we have tested stand above 

 the best glasses that we have seen hitherto. 



E, -yVth (4 guineas), is a dry glass, and is in magnifying 

 power somewhere between Hartnack's No. 9 (new glass with 

 correction), and his immersion No. 10. The E's that we have 

 examined are unquestionably superior to Hartnack's No. 9. 



All these three glasses (D, E, E) have a flat field, so that 

 the peripheral objects appear as sharp as those in the centre of 

 the field. 



Immersion J (trth, £7 4s.), Im. K (Voth, £10), and Im. L 

 {^th, £13 10s.), which I have examined, appear to me as good 

 as any corresponding immersion lens of English or continental 

 manufacture. E. Klein. 



Celebration in honour of Theodor Schwann, the founder of 

 the Cell-theory. — Professor Schwann, of Liege, has now 

 reached the fortieth year of his professorship. His past and 

 present pupils are about to celebrate this event by presenting 

 him with a marble portrait-bust of himself and with ad- 

 dresses of congratulation. Professor Edouard van Beneden, 

 of Liege, or the editors of this Journal will be glad to receive 

 addresses from English scientific bodies or from individuals 

 for presentation on the occasion. 



