THE IMMERSION SYSTEM. 83 



it was not recognized that additional aperture was also 

 obtained. In 1877 Mr. J. W. Stephenson demonstrated 

 that as the aperture of an objective increased with 

 the increase in the refractive index of the immer- 

 sion fluid great practical advantage would result 

 from using instead of water a homogeneous fluid; 

 that is, one not merely of the same refractive index, 

 but also of the same dispersive power as the glass 

 of the front lens of the objective. This sugges- 

 tion was immediately acted upon by Professor Abbe, 

 and in December, 1877, the, first objective on the new 

 system was issued from Zeiss's workshop, giving an in- 

 crease in aperture of upwards of 50 per cent, over a dry 

 objective of equal angle. In addition to increase of 

 aperture, the use of a homogeneous fluid gives a pre- 

 viously unlooked-for advantage that it is possible to 

 correct a "homogeneous immersion" objective with 

 more facility than one which works in such media as air 

 and water, both of which differ considerably in refrac- 

 tive and dispersive power from the glass of the lenses. 

 With air, or even water objectives, there is a large 

 amount of aberration affecting the pencils on their 

 passage from the radiant to the medium of the front 

 lens, which bears a considerable ratio to the total 

 spherical aberration within the objective, and in the 

 case of wide angles increases disproportionately from 

 the axis outwards. This can only be corrected by a 

 rough method of balancing, that is, by introducing an 

 excess of opposite aberration at the posterior lenses. 

 An uncorrected residuum, rapidly increasing with larger 

 apertures, is then left, and this appears in the image 

 amplified by the total power of the objective, so that 

 with a non-homogeneous medium there is a maximum 

 angular aperture which cannot be surpassed without 

 undergoing a perceptible loss of definition, provided 

 working distance is required. If we abolish the an- 

 terior aberration for all colours, by an immersion fluid 

 which is equal to crown-glass in refractive and dis- 

 persive power, the difficulty will be at once overcome. 

 If, for instance, we have an objective of 140 in glass 

 (= 1*25 N.A.) and water as the immersion fluid, the 

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