October. 1912. 



KNOWLEDGE. 



397 



by lantern slides was given bj' the Secretary of the Selborne 

 Society on the successful experiment in Bird Protection made 

 during the last eight or nine years in the Brent Valley Bird 

 Sanctuary and the nesting boxes which have been designed in 

 connection with it. 



PHOTOGR.AIMiV. 



By Edg.ar Se.n'ior. 



PHOTOMICROGRAPHY WITH HKiH POWTIRS.— 

 In the last issue of " KNowi.KDGii," when dealing with 

 medium power photo-micrography, we endeavoured to show 

 that magnification beyond the degree necessary to observe 

 distinctly all that it was possible for the objective to define, 

 was practically useless, .and that it was the aperture that 

 determined the essential qualities required in the lens. We 

 have an example before us which illustrates this admirably. 

 Two objectives, one of which is a quarter of an inch having an 

 aperture of -95 N. A., the other a fiftieth of an inch, of -98 N.A. 

 Now, the former, according to theory, would be able to resolve 

 lines or dots when so close together that ninety thousand two 

 hundred and sixty were contained in the linear inch, and the 

 magnifying power at ten inches would be forty diameters. 

 With the latter the resolving power would be ninety-three 

 thousand one hundred, and its magnifying power five hundred 

 diameters at the same distance. The magnification is. there- 

 fore, out of all proportion to the resolving power, as without 

 any eyepiece at all. the size of the image would be e(iual to. 

 " and theoretically greater than," that obtained by means of the 

 quarter-inch when used with a ten ocular. It, therefore, becomes 

 difficult to understand what was gained by the use of an objective 

 of this focus, leaving out of consideration the inconvenience 



microscopy, we see how essential it is that an objective should 

 possess a wide one, when intended for use in the observation 

 of minute objects such as bacilli, or the study of the 



FiGfRE 431. Bacilli, photographed with a Zeiss 2 nmi. homo| 

 apochromatic objective N.A. 1-40 and a four projection ocular 



arising from its close working distance. However, as this latter 

 objective has a larger aperture, there should be a gain in that 

 respect. Realizing, then, the great value of aperture in 



FiGURi-; 432. 



The liu of a millimetre divisions of a stage micrometer 



X 1900 diameters. 



structure in diatoms. Owing to the researches of Professor 

 Abbe, in connection with the defining (resolving) power of the 

 microscope, we have the general formulae 2 X N.A. as 

 expressing it. That is. twice the wave length of light employed 

 multiplied by the numerical aperture of objective. It should 

 also be evident that anything which diminishes X increases the 

 resolving power of the lens, and as the wave length of light 

 is less in a highly refracting niediuiTi 

 than it is in air, we can understand ihe 

 advantage gained by the use of homo- 

 geneous immersion objecti\es. The 

 immersion fluid usually employed is 

 cedar- wood oil, having a density of 1-515. 

 and in order to thoroughly miderstand 

 its use, we must consider the equation 

 given by Professor Abbe, which dett^r- 

 mines the value of the numerical 

 aperture ; thus N.A. = n sin u, in which 

 n stands for the density of the mediinu 

 in which the front lens of the objective 

 is immersed, and sin /i represents tlie 

 sine of half the angular aperture. Suppose 

 a dry objective having an angular aper- 

 ture of 134° 9' be the one considered, air 

 being the medium in front of the lens, 

 the index of refraction is unity, therefore 

 n = l. Half the angle of aperture is 



— - — =67° 45'. Referring to a table 



of sines we find that the sine of 67° 45' 

 is -92554, therefore the N.A. = 1 

 X 0-92554. Supposing that it were 

 possible to use this same objective 

 with cedar-wood oil between its front 

 lens and the cover glass, then n would 

 equal 1-515 and the N.A. = 1-515 

 X 0-92554 = 1-40. We thus see the 

 t;reat advantage of oil immersion, and 

 how the N..'\. determines the essential 

 iiualities of the objective with regard 

 to its ability to resolve fine structure. 

 Photography also places a still further 

 power in our hands, as by its means 

 and the employment of apochromatic 

 objectives, we are able to make use 

 of blue light, and so obtain greater 

 resolution still, owing to their shorter 

 wave length. That the use of high 

 powers requires special care to be taken in the adjust- 

 ment of the apparatus is generally well known ; but more 

 particularly is this the case in photo-micrography. In order 



;eneous immersion 

 X 1900 diameters. 



