THE EXCLUSION OF THE SECONDAKY SPECTKUM 365 



two double backs. A general diagram of their mode of construction 

 is given in fig. 318. 



So long as crown glass was employed in their manufacture, and 

 the anterior front lens was a hemisphere, it appeared that N.A. 1*25 

 to 1*27 was the aperture limit they could be made to reach. 

 Messrs. Powell and Lealand, however, by making the anterior front 

 lens greater than a hemisphere, increased the aperture of a jV-inch 

 objective to 1'43 KA. 



This front, from being greater than a hemisphere, presented 

 difficulty in mounting ; this was at first overcome by cementing its 

 plane surface to a thin piece of glass, which was then fixed in the 

 metal. Eventually, however, this form of construction was changed 

 by these makers in a very ingenious manner ; so to speak, they 

 entirely inverted the combination, and accomplished the end by 

 making the front of flint. By this means they obtained apertures 

 which have not as yet been equalled by any other makers, reaching 

 in a ^, a r V, and a ^ a IS". A. of 1'50 out of a theoretically possible 

 aperture of 1'52. Professor Abbe has since, it is true, made an 

 objective with a numerical aperture of 1'63, but this requires the 

 objects to be mounted and studied in a medium of corresponding 

 refractive index, and consequently, in the present state of our know- 

 ledge of the subject of media, not applicable to the investigation of 

 ordinary organic structures certainly not of living things. 



These objectives fully occupied the microscopist until 1886, when 

 the most important epoch since the discovery and application of 

 achromatism was inaugurated. 



We have already pointed out in detail x that it was the great 

 defect of the ordinary crown and flint achromatics that two colours 

 only could be combined and that the other colours caused out-of- 

 focus images, which appeared as fringes round the object. This was 

 what was known as the residuary secondary spectrum. 



In like manner, it has been shown that it was not possible in the 

 flint and crown achromatic to combine two colours in all the zones 

 of the objective, so that if two given colours are combined in the in- 

 termediate zone they will not be combined in the peripheral and 

 the central portions of the objective. 



These phenomena, it has been pointed out, 1 arise from what is 

 known as the irrationality of the spectrum. To correct this we have 

 seen that Drs. Abbe, Schott, and Zeiss directed their attention to 

 the devising of vitreous compounds which should have their dis- 

 persive powers proportional to their refractive indices for the various 

 parts of the spectrum. Only by these means could the outstanding 

 errors of achromatism be corrected. 



It is therefore a fact that the old flint and crown objectives, 

 whether for the microscope, the telescope, or the photographic 

 camera, are, strictly speaking, neither achromatic nor aplanatic. 



Glass whose properties far more nearly approximated the theo- 

 retical requirement than any previously attainable having been 

 manufactured by the Jena opticians, 2 Abbe was able to produce 

 objectives entirely cleansed of the secondary spectrum. From calcu- 

 1 Chapter I. * Chapter II. 



