88 Modern Microscopy 



above, in a fluid form is made by mixing a small proportion 

 of malachite green in glycerine in a trough. The light from 

 the illuminant that is to be used is examined spectroscopi- 

 cally through the medium in the trough, and the coloured 

 fluid is added until the red end of the spectrum is 

 absorbed ; if this be done exactly a minimum of loss of 

 light occurs. 



Another excellent screen is produced by making a satur- 

 ated solution of acetate of copper, but a trough with an 

 opening of at least I- inch back to front is necessary to 

 obtain an effective colour with this fluid. 



For photography, discs of light and dark yellow and 

 various shades of blue glass are constantly employed. The 

 great desideratum in a light screen is that it shall pass a 

 large quantity of light. No doubt, mixtures could soon be 

 made that would produce monochromatism, if only great 

 transparency were not of importance. 



Mr. Nelson states* that monochromatic blue light ' makes 

 a difference of about 14 per cent, in the case of low aper- 

 tures, but beyond those of 0"9 N.A. its influence in increas- 

 ing resolution is so small as to be hardly worth taking into 

 account. What it does effect is the sharpening and clearing 

 of detail already resolved.' 



This is no doubt because only a small part of the first 

 diffraction image, seen when looking at the back of the 

 objective with the eyepiece removed, is utilized by the lens, 

 and that this part consists of blue light whether it be blue 

 or white light that is employed for illuminating, so that in 

 both cases there are the same materials for resolving the 

 detail of the object, and the only difference is that there is 

 a haze of light of lower refrangibility (or greater wave-length) 

 also present, which forms a luminous veil over the whole 

 field. It is this veil which is removed by using monochro- 

 matic light, and therefore the effect is to sharpen and clear 

 the detail that is already resolved. 



* Jouriial of the Royal Microscopical Society, 1893, p. 15. 



