FOR THE MICROSCOPE. 95 



represents the natural sine of half the angle of aperture of the 

 objective in air. The refractive index of air being taken as i, 

 water is 1*33, and the kind of oil usually employed is i'52. On 

 this basis the greatest possible theoretical angle in air — 180° — is 

 equalled by 97*^ in water and 82^ in oil ; that is to say, as many 

 rays pass into the objective through oil with an aperture of 82°, 

 and through water with 97 ''j as can be received through air in a 

 " dry " objective with an angle of 180°. 



The advantage, therefore, of the immersion system is obviously 

 very great. The highest results which the present condition of 

 optical science has placed within the reach of the microscopist are 

 obtained, so far as the present writer is aware, by the use of the 

 apochromatic system of homogeneous immersion lenses construct- 

 ed by Carl Zeiss, of Jena, from formulae of Prof. Abbe, and 

 specially by an objective with an aperture of i'6o N.A , with 

 which an immersion fluid of higher refractive index than oil is 

 used. There does not appear to be any scientific bar to the ' con- 

 struction of objectives of a yet increased aperture. The difficulty 

 consists in the discovery of a suitable immersion liquid, which 

 should have an index of i"8 or 1*9 to make any real advance on 

 present attainments. Glass for the front lens and cover-glasses of 

 the corresponding index could be made without difficulty so as to 

 preserve the homogeneity. 



Regarding the relative merits of the best German and English 

 made objectives, the present writer is not aware of any superiority 

 in any way in the English make, nor of any inferiority. The 

 German glasses are first-rate in performance and in workmanship 

 throughout ; whilst in the case of some of the lower powers (half- 

 inch to quarter-inch) the cost is from j[^2 to £^t^ each less than 

 their English rivals. The cost in all cases increases rapidly as the 

 numerical aperture approaches the limits. A one-inch Zeiss' 

 apochromatic objective with 0*30 N.A. costs ^£6 ; a half-inch of 

 0*65 N.A., jQ"] ; and the special i/io lens, above referred to, of 

 I '60 N.A., costs ^40 : while the best English lenses cost ;£io for 

 a half-inch of 0*64 N.A., and jQ^o for a i/ioth with 1-50 N.A. 

 These high prices are a secondary consideration where scientific 

 investigations demand their use ; but they are, without doubt, a 

 bar to the extensive use of the new lenses among ordinary workers. 



