NOTES AND MEMORANDA. 173 



lamella or supporting layer (Stiitzlamella) ; it is very thick in the 

 wings, and serves for the attachment of the muscles. 



The author concludes with a discussion of the affinities of Tetra- 

 pteron, which he considers to hold an intermediate place between 

 Polypes and Medusas. 



The Algae of the "White Sea. — This paper, by Dr. C. Gobi, 

 in the Memoirs of the St. Petersburg Academy,* is the first detailed 

 account of the algfe of the White Sea. The species are principally 

 those found throughout the Arctic Ocean ; but Dr. Gobi remarks that 

 the vegetation of the southern part of the White Sea has a more 

 northern character than that of the northern part, which is explained 

 by the statement that many forms of Western Europe which make their 

 way to the northern part do not extend to the southern part. Dr. Gobi 

 unites a considerable number of species considered by Agardh and 

 others to be distinct, even regarding Bliodomela lycopodioides as a 

 form of B,. suhfusca and PolijsipJionia arctica as a variety of P. varie- 

 gata. Bhodophyllis veprecula, Ag., is referred to It. dichotoma, Le- 

 pechin. The new species and varieties observed and studied by Dr. 

 Gobi amount to nine, and the total number of species gathered to 

 seventy-six. 



The paper contains valuable references to the species of Euprecht 

 in the Academy's herbarium. 



Achromatic Lenses. — Mr. E. M. T. Tydeman, of Liverpool, has ob- 

 tained provisional protection for an invention which we describe nearly 

 in his own words, as appearing in the printed specification: — "My 

 invention consists of improvements in the construction of compound 

 achromatic lenses suitable for use in Microscopes and other optical 

 instruments, and is intended more completely to eliminate the large 

 irrationality or want of correspondence between the coloured spaces in 

 the various spectra (secondary spectrum), and to render the lenses more 

 perfectly achromatic. It consists in forming the lenses, not as hitherto 

 by the union of lenses made of different kinds or species of glass or 

 other refractive media, but of one kind or species only, yet of different 

 densities and refractive powers, in which the irrationality or unequal 

 refraction of the coloured rays is not so great. I therefore construct 

 my improved achromatic lenses with two or more glasses made from 

 material of the same kind or species of glass (such as that known as 

 flint glass, which is capable of being made of varying density), but of 

 different densities or refractive indices; and I also use flint glass 

 lenses in lieu of the usual crown or plate glass lenses in achromatic 

 object-lenses. For a Microscope object-glass — often composed of two 

 or more approximately achromatic lenses, or set of lenses, either in 

 contact or nearly in contact — I sometimes make one of the several sets 

 of compound lenses of one refracting medium, such as flint glass, and 

 the other set or sets, or single lenses, of a different refracting medium 

 or media, or I use single lenses of crown, or flint, or any other 

 substance in combination, though not necessarily in contact with an 



* 'Mem. Imp. Acad. Sc. St. Petersburg,' vol. xxvi. (1878); 'Amer. Jour. Sci. 

 and Arts,' ser. 3, vol. xvii. (1879) p. 71. 



