MEMORANDA. 87 



itself, so that if it were possible to use an objective of 180° 

 of aperture, the effect of balsam-mounting would reduce it at 

 once to 82°, and allowing for all possible variations of the 

 refractive powers of the balsam and cover, I have no hesita- 

 tion in affirming that any object mounted in the usual manner 

 in this medium, has never been seen with an angle greater 

 than 85° ; but in all probability the extreme limit has been 

 about 78°. This statement is not the result jf mere hypo- 

 thesis, but admits of ocular demonstration, by experiments that 

 will prove it at least half-a-dozen different ways, and is so 

 true in theory, that to endeavour to disprove it will be to 

 take the difficult course, of attempting to undermine the 

 ground upon which I have taken my stand, by denying the 

 first laws of refraction upon which my assertion is based. 



Professor Bailey has, no doubt, experienced the advantage 

 of the utmost extent of aperture that can be obtained, in that 

 particular department of investigation, in which he has so 

 eminently distinguished himself; and I am willing to admit, 

 that if the highest powers are to be used only for viewing 

 thin and flat objects like the Diatomacece, the aperture may be 

 as near to 180° as may be practically convenient for this 

 especial purpose ; but considering all the requirements, and 

 perhaps more useful applications of the object-glass, I am 

 still of opinion that beyond 150° there is no real advantage to 

 be gained. I have expended much time, and taken special 

 delight in the cultivation of the largest apertures, and possess 

 an assortment ranging up to the greatest possible limit, and 

 I can even now bring out striae with 150° as readily as 

 with anything beyond it, with the positive advantage of a 

 greater distance between the front lens and object. Some of 

 the phenomena described in my communication to the present 

 Journal are extremely severe tests of all the good qualities of 

 an object-glass, and yet I have had some, whose performance 

 is unrivalled upon a difficult dlatomaceous test, repeatedly 

 break down and fail in their effective duty, when applied to 

 the investigation of plant-circulation, from the fact of their 

 possessing too much aperture. — F. H. Wenham. 



On the Structure of the froud of Polysiphonia fastiginta. — 1 he 



frond oi Pohjsiphonia fastigiata, bearing antheridia, consists of 

 a mass of transparent matter, in which are imbedded coloured, 

 elongated cells or siphons. Tliese are so arranged side by 

 side in successive rows as to surround a central hollow passing 

 through the whole extent of the frond. Each row of siphons 

 with its hyaline matrix forms a kind of ring or section of a 

 tube, and under pressure has a tendency to detach itself from 



