580 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The spherical Foraminifer Orbulina universa is 600 fx in diameter, 

 consequently a depth of vision of 300 fi is necessary to see the whole 

 under one focussing. The 1^ objective and A eye-piece magnifying 

 together 30 diameters, will just suit this, provided it does not possess 

 an apertiu-e exceeding 0*17, but if we wish to photograph this 

 spherical body a much lower objective than the 1^ in. must be 

 employed, as the focal depth of this objective is not higher than 

 86 fx. 0. universa affords a good proof of the accuracy of Prof. Abbe's 

 figures. Under the Ih in. objective of • 14 N. A. the spheres are 

 splendidly seen, and the same may be said of the 2 in. of • 14 N.A. 

 and B ocular, but when the picture is thi'own upon a ground-glass 

 screen the want of penetration is soon apparent, for it is only when 

 the amplification of the picture has been reduced to rather less than 

 10 diameters that a satisfactory result is obtained. 



Similar illustrations may be offered of the higher power objectives. 

 The larger si)ecies of Polycistina require a depth of 75 jx to show 

 them distinctly, whereas a 1-2 in. objective of • 34 N.A. in air, when 

 used with the A eye-piece, to produce 100 diameters of amplification, 

 possesses but 10 • 6 micras. 



A l-6th objective, magnifying 300 diameters, loses exactly 

 1/xin depth between 0"60 N.A. and 1*2 N.A., so that while the 

 spores of PeniciUium glaucum (diameter of spores 3 //,) could be 

 photographed with the former, it would be impossible to obtain 

 perfect sharpness with the latter. 



The figures in the table for the l-12th in. and l-20th in. objectives 

 are equally confirmed by the results obtained in practice. The short 

 diameter of Bacterium termo may be taken as 0*8 /a, requiring a 

 penetration • 4 /a to yield a clear picture, and this is obtainable by 

 using a homogeneous l-12th in. of 1 • 10 N.A. to produce an amplifica- 

 tion of 600 diameters. A l-20th in. objective, magnifying 1000 

 diameters, although producing a fairly sharp picture to the observer's 

 eye, cannot produce an equally sharjJ image on a prepared plate, as 

 the focal depth of such an objective will only approximate to 

 0'S7 II, and this statement is borne out by the photograjjhs published 

 by Dr. Sternberg, in his translation of Magnin's ' Treatise on the 

 Bacteria,' wherein those pictures taken with a Beck's l-5th in. are 

 much clearer, though smaller, than the plate taken with Zeiss' 1-1 8th. 

 There is more detail in the latter, and here comes in the value of 

 amplification and apertiu-e. 



Reference is also made to the increase in the depth of vision in 

 direct proportion with the refractive index of the mounting medium. 

 The great gain in stereoscopic efiect, on objects mounted in a medium 

 of high refractive index, has led Mr. E. Ward, of Manchester, to 

 mount opaque objects in balsam, with extremely good results. 



Value of Photography in Microscopical Investigations."^ — 

 R. Hitchcock discusses the question whether photography affords a 

 means of illustration or demonstration in any wise equal or superior 

 to drawing by hand. On the one side it may be said that a photo- 



* Amer. Mon. Micr. Journ., iv. (1883) pp. 33-4. 



