MEMORANDA. 61 



— only this is transparent, while Cristatella is, as I said, 

 feathery-looking, like, as M. Gervais has observed, hits of 

 Chenille — it is impossible, I think, to find a more apt simili- 

 tude. And now that you have once seen one specimen, you 

 will have little difficulty in being able to discover any amount 

 of others. 



The extreme beauty of this Polyzoon, the fact of its con- 

 tinuing in an exserted state, even under rough treatment, 

 the transparency of the coenoecium rendering an anatomical 

 examination so easy and satisfactory, must always make 

 Cristatella a great favorite, and one of the most prized and 

 beautiful of all the beautiful forms of aquatic life. — Houghton. 



Angle of Aperture. — Observing in the ' Microscopical 

 Journal' of July last, an article on the measurement of 

 angular aperture of lenses from the pen of P. Gray, Esq., 

 and deeming its recommendations based upon determining 

 these observations irrespective of any special apparatus, thus 

 seemingly presenting facilities to individuals who may neither 

 have such in possession, or ready access thereto, I have ex- 

 amined the practical merits of the method in question, in 

 contrast with the known angular aperture of lenses in my 

 possession determined by other means, as No. 1 = 25° ; 

 No. 2 = 63°; No. 3 = 91°; No. 4 = 129°; the following 

 results being obtained, exercising all ordinary care. 



Lights apart 7 inches 



Distauce of lens 1675X2 



Lights apart 15'50 inches 

 Distance of lens 23 



The results by this lens were very variable, though not so 

 by the usual several feet radius. 



No. 3. 



T . . . ort „ . , Decimal Number. Logarithm. Tanqent. 



Lights apart 30-5iDchea = . 95Q6 = g? / 



Distance of lens 14£x2 2 



Angle of aperture 87 - 06 

 Two other experiments gave similar results ; one gave 90°. 



