MEMORANDA. 89 



with the axis of the microscope, bj rack-and-pinion movement, 

 as well as varied in its distance from <he stage by the same kind 

 of motion. It may be further observed that when the angle, 

 which the axis of the condenser makes with the axis of the 

 microscope, is greater than half the angle of aperture of the 

 object-glass, the black-ground illumination is produced in the 

 most perfect manner. Provided this condenser be attached 

 to the stage of the microscope by a circular arc divided into 

 degrees, the angle of aperture of the object-glass under all 

 conditions can be accurately ascertained, for the limit of aper- 

 ture will be when the illuminated field is just passing into 

 the black-ground illumination. — J. D. Sollitt, Grammar- 

 School^ Hull. 



ITIicroscopical Examinatioii of Deep Soundings from the Atlautic 



Ocean. — ^The soundings examined were as follows : — 



1080 fathoms, Latitude 42= 04' North, Longitude 29^ OO' West, Julj- 25, 1853. 

 1360 ,, ,, 44M1' ,, ,, 24^35' ,, ,, 18 ,, 



1580 ,, ,, 49= 56' 30" ,, 13= 30' 45" Aug. 22 ,, 



1800 ,, ,, 47=38' ,, ,, 09=08' ,, No date. 



2000 ,, ,, 54=17' ,, ,, 22=33' ,, ,, 



As these soundings are believed to be the deepest ever 

 submitted to microscopic examination, and were obtained at 

 localities far remote from those previously noticed, they were 

 studied vei'y carefully, and the following are the facts ascer- 

 tained : — 



1. None of these soundings contain a particle of gravel, 

 sand, or other recognizable unorganized mineral matter. 



2. They all agree in being almost entirely made up of the 

 calcareous shells of minute, or microscopic Foraminiferae 

 i^Pohjthalamia, Ehr.), among which the species of Globigerina 

 greatly predominate in all the specimens, while Orhulina uni- 

 versa, D'Orb., is in immense numbers in some of the sound- 

 ings, and particularly abundant in that from 1,800 fathoms. 



3. They all contain a few specimens of non-parasitic or 

 pelagic Diatoms, among which Coscinodiscus lineatus, C. ex- 

 centricus, and C. radiatus of Ehrenberg, are much the most 

 abundant. 



4. They all contain a few siliceous skeletons of Polycis- 

 tineae, among which are several species of Haliomma, Litho- 

 campe, 6cc. 



5. They all contain spicules of sponges, and a few speci- 

 mens of Dictyocha fibula, Ehr. 



6. The above-mentioned organic bodies constitute almost 

 the entire mass of soundmgs, being mingled only with a fine 

 calcax'eous mud derived from the disintegration of the shells. 



