equally sharp. The quality of the image will therefore be always uniformly perfect 

 whether white or monochromatic light be employed for illumination. 



Again, in the older series complete colour correction is obtained for one 

 zone of the objective only, a marked deterioration being observable towards the margin 

 and the centre of the aperture, whilst in the apochromatic lenses uniformly perfect 

 colour correction is obtained for a 1 1 zones alike. Consequently, in using ABBE'S 

 test plate for instance, scarcely any more colour is perceived with extreme oblique il- 

 lumination than with semi-lateral or with central light. 



Finally, with the ordinary achromatic objectives, 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 objectives, whereby the focal differences of the 

 various sections of the spectrum are reduced to fractions varying from 1 / 1 to l / lo of their 

 original magnitude, i. e., are practically eliminated, and this, we will repeat, is 

 done in equal degree 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 objectives are obvious. Their 

 considerably increased power of concentrating the light with ordinary visual observ- 

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

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

 lishes 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 large aperture and comparatively great working distance, 

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

 Therefore the marginal and central portions of the field do not appear sharply 

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

 screw. 



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

 the use of oculars of very high power, while still retaining satisfactory 

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



') Achromatisa t ion of a higher order in our sense of the term is both theoretically 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 indicated 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 realised 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 utilise it for pur- 

 poses of trade advertisement of any other casual improvement in colour correction should be dis- 

 countenanced. 



Qazl ci, Stnet. 



