ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1033 



&c. Thus taking the 1/4 above and deducting 3 per cent, from the 203, 

 we get 197, which, divided by 5, gives 39 -4, a result very near the truth. 



A certain 1/8 gives a combined power of 450, deduct 6 per cent. = 423, 

 divide by 5 = 84*6, the actual being 85. For short bodies of 6^ in., or 

 Continental size, a different percentage must be employed. The following 

 gives fair results: — 2 per cent, for 1/2, 4 per cent, for 1/4, 6 per cent, for 

 1/6, 8 per cent, for 1/8, and 10 per cent for 1/12. 



Method of Intensifying the Resolving Power of Microscope Ob- 

 jectives.* — Mr. G. D. Hirst describes a simple way of vastly improving 

 the definition of objectives on close-lined test objects, which has lately come 

 under his notice. The credit of the discovery is due to Mr. Francis, of 

 Sydney. 



Take a valve of, say, Amphipleura pellucida, and, having got the best 

 results obtainable with mirror and condenser, let the analysing prism 

 belonging to the polarizing apparatus be placed over the eye-piece, and 

 rotated until it darkens the field, which it will do, though not to the same 

 extent as when used with the polarizing prism. On carefully focusing the 

 diatom, the lines will show themselves with an extraordinary increase of 

 definition. Valves that without the aid of the prism only show a washy 

 sort of resolution, will now show the lines as black as the bars of a gridiron. 



On P. angulatum by central light the result is also splendid. The same 

 effect can also be obtained, though perhaps to a slightly inferior degree, 

 with the objective, or, as it is placed in some stands, in a sliding box in the 

 body of the Microscope ; in the latter case, as it cannot be rotated, the valve 

 of A. pellucida should lie horizontally. For general purposes, it is better 

 for the prism to fit over the eye-piece, as besides giving better definition in 

 that position, with a diatom like P. angulatum and prism over the objective, 

 the diffraction spectra would be cut out of the top and bottom of the back 

 lens and the effect spoiled. Of course, in the case of A. pellucida, with the 

 valve lying horizontally, it does not matter, as the dioptric ray and single 

 spectrum are not cut off in any way by the prism or the box in which it is 

 set. The prism has the effect of greatly diminishing the light of the 

 dioptric beam ; at the same time it scarcely touches that transmitted by the 

 diffraction spectra. 



The application of the prism will not of course make an objective 

 resolve a test beyond the reach of its aperture ; but it often happens that in 

 the case of close-lined objects we can see the spectrum at the back of the 

 objective when the lines cannot be seen in the object itself. It is then that 

 the prism shows its power, as its use will at once bring out the lines with 

 the greatest ease and sharpness. 



Mr. E. M. Nelson f found, while investigating the matter, that the 

 diffraction spectrum of A. pellucida (illuminated by oblique beam from oil- 

 imm. achromatic condenser, and with a water-imm. 1/12) showed all the 

 green, but no red. On examining the spectrum through the analysing prism 

 without an eye-piece he found that when the prism was in a line with the 

 dioptric beam and the diffx'action spectrum, the brightness of the green was 

 intensified. On replacing the eye-piece, and viewing the image through the 

 prism used above the eye-piece, as directed by Mr. Hirst, there could be no 

 doubt that the transverse strire were much sharper and blacker than when 

 viewed without the prism. The prism must, of course, be kept in a line 

 with the dioptric beam and the diffraction spectra. Should the prism be 

 turned across, even if it does not cut off aperture, the definition will be 

 impaired. 



* Eug. Mech., xlvi. (1S87) p. 232. t Ibid-, p. 254. 



