. The book of photography; practical, theoretical and applied. \e ob-. Fig. 690.—Eosss Standard Microscope. jectives ; they are designated as 2 in., 1in., i in., J in., | in., i in., and so on, orby the (Continental measures, 24, 16, 12,8, 6, 4 mm., etc. These numbers repre-sent, not the focal distances of the lenses,but the focal distances of simple lenseshaving equivalent magnifying are now always constructed insuch a way that they are achromatic ; thatis, they consist of a combination of lensesof flint glass and of crown glass, the raysof light refracted by the latter bein
. The book of photography; practical, theoretical and applied. \e ob-. Fig. 690.—Eosss Standard Microscope. jectives ; they are designated as 2 in., 1in., i in., J in., | in., i in., and so on, orby the (Continental measures, 24, 16, 12,8, 6, 4 mm., etc. These numbers repre-sent, not the focal distances of the lenses,but the focal distances of simple lenseshaving equivalent magnifying are now always constructed insuch a way that they are achromatic ; thatis, they consist of a combination of lensesof flint glass and of crown glass, the raysof light refracted by the latter being 526 THE BOOK OF PHOTOGRAPHY. used, when an eyepiece giving a very hiuhamplification, or when other than mono-chromatic light is used, then these ob-jectives break down and show colourfringes round the object or on the limit ofthe field. This is due to the fact thatly two colours of the spectrum arein one point, and there is a residualcalled secondary spectrum un-cted for. This defect, however, is
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