112 



Mr. F. H. Wenham on a new Formula [Jan. 16_, 



appears to have been used merely with the intention of gaining an 

 increase of power, in ignorance of any principle, and without even a 

 knowledge of the value of angular aperture. 



At this time the late J. J. Lister tried a number of experiments, and 

 discovered the law of the aplanatic focus, and proved that, by separating 

 lenses suitably corrected, there were one or two positions in which the 

 spherical aberration was balanced. This was explained in a paper read 

 before the Eoyal Society in 1829. In the year 1831 Mr. Eoss was em- 

 ployed to construct the first achromatic object-glass in accordance with 

 this principle, which performed with " a degree of success never antici- 

 pated." 



Mr. Eoss then discovered that, after he had adjusted the interval of his 

 lenses for the aplanatic focus, that position would no longer be correct if 

 a plate of thin glass was placed above the object ; this focus had then to 

 be sought in a different plane, and the lenses brought closer together, in 

 order to neutralize the negative aberration caused by covering-glass of 

 various thickness. From this period the " adjustment " with which all 

 our best object-glasses are now provided became established. Fig. 1 is 

 the form of object-glass used at this time, consisting of three plano- 

 concave achromatics, whose foci were nearly in the proportion of 1, 2, 3. 



No greater angle than 60° could be obtained with this system in a 

 J-inch objective (the highest power then made), for reasons apparent in 

 the diagram. The excessive depth of curvature of the contact-surfaces 

 of the front pair is unfavourable for the passage of the marginal rays ; 

 the softness of the flint glass forming the first plane was also objection- 

 able. In the year 1837 Mr. Lister gave Mr. Eoss a diagram for an im- 

 proved " eighth," having a triple front lens in the form shown in fig. 2. 

 By this the passage of extreme rays was facilitated ; and in order to 

 diminish the depth of curvature, a very dense glass was used, having a 

 specific gravity of 4-35L Faraday's glass, having a density of 6*4, had 

 been previously tried, but was abandoned on account of a difficulty in 

 working it. The polished surfaces of both these qualities of dense glass 

 speedily became tarnished by exposure to the air ; and thus the dense 

 flint concave could only be employed in a triple combination, that is, 

 when cemented between two lenses of crown glass : this form of front 

 was kept a trade secret, and was not published in any work treating of 

 the optics of the microscope. The front incident surface of the flint of 

 the middle pair was made concave, in order to reduce the depth of the 

 contact ; and for this reason only, as that surface has but little influence 

 in correcting the oblique pencils, or in producing flatness of field, and 

 may be a plane with an equally good or better result. " Eighths " of this 

 form with angles of 80° were made, and remained unaltered till the year 

 1850, when larger apertures were called for, and Mr. Lister introduced 

 the triple back lens. 



The necessity for this will be seen by the diagram (fig. 2), which shows 



