correction is obtained for all zones alike. Consequently, in using ABBE'S test plate for 

 instance, scarcely any more colour is perceived with extreme oblique illumination than with 

 semi-lateral or with central light. 



Finally, with the ordinary achromatic lenses, even within the zone of their most 

 complete colour correction, only two colours at a time can be united in one point and 

 only for this pair of colours can the images fall on the same spot, considerable focal 

 difference remaining for all others. But, as already mentioned, three colours are 

 brought to a focus in the apochromatic lenses, whereby the focal differences of the various 

 sections of the spectrum are reduced to fractions varying from y ? to l / w of their original 

 magnitude, i. e., are practically eliminated, and this, we will repeat, is done in equal 

 degrees for all zones of the objective. The images due to different colours, each one 

 individually corrected, are thus rendered perfectly coincident and collectively form the final 

 image 1 ). 



The practical advantages of these new lenses are obvious. Their con- 

 siderably increased power of concentrating the light with ordinary visual obser- 

 vation or with any other mode of application - and this under all possible 

 conditions of illumination, central or oblique, white or monochromatic - - esta- 

 blishes their acknowledged superiority over all earlier constructions, both with 

 respect to optical power and diversity of applicability. 



The natural colours of objects, even delicate differences of tint, are 

 faithfully reproduced by these objectives. The images are nearly as sharp 

 close to the margin of the field as they are in the centre. 



It is true that, owing to their high aperture and comparatively great working distance, 

 even in these objectives a slight curvature of the image cannot be entirely avoided. There- 

 fore the marginal and central portions of the field do not appear sharply in 

 focus simultaneously, but have to be focused in succession by means of the micrometer 

 screw. 



As a result of their great light-gathering power these objectives admit of 

 the use of eye-pieces of very high power, while still retaining satisfactory 

 precision and brightness of the image and thus giving high magnifying powers 



J ) Achromatisation of a higher order in our sense of the term is both theore- 

 tically and practically quite another thing than a mere improvement, in degree, of ordinary 

 achromatism, such as would result from a diminution of the secondary spectrum while yet 

 only two colours are united; or than achromatisation embodying the principles here indi- 

 cated but limited to one particular zone of the objective, as would, for instance, be the 

 case if suitable glasses were introduced in constructions of ordinary type. Professor 

 ABBE has introduced the word "apochromatic" as a definite technical term for this 

 particular kind of achromatism, so long familiar to scientists as a theoretical idea but only 

 recently realized practically. In order to avoid ambiguity it is desirable that this expression 

 should retain its original meaning as clearly defined by its author, and any attempt to 

 utilize it for purposes of trade advertisement of any other casual improvement in colour 

 correction should be discountenanced. 



(Bavl 



