Mr. Wenham's new Formula for a Microscope Object-glass, 225 



matic object-glass in accordance with this principle, which per- 

 formed with " a degree of success never anticipated." 



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 aberra- 

 tion 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 achro- 

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



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

 in a g-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 pas- 

 sage of the marginal rays ; the softness of the flint glass forming 

 the first plane was also objectionable. In the year 1837 Mr. Lister 

 gave Mr. Eoss a diagram for an improved " eighth," having a triple 

 front lens in the form shown in fig. 2. By this the passage of ex- 

 treme rays was facilitated; and in order to diminish the depth 

 of curvature, a very dense glass was used, having a specific gravity 

 of 4-351. Faraday's glass, having a density of 6*4, had been pre- 

 viously 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 com- 

 bination, 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. 

 Thejront 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 that the contact-surfaces of the back achromatic are too 

 deep, thus giving great thickness to the lens and limiting its dia- 

 meter : dense flint would have remedied this to some extent ; but 

 its habihty to tarnish rendered its use in a pair objectionable. The 

 highest density at this time known, quite free from this defect, was 

 3*686. By means of the triple back, the final corrections were ren- 

 dered less abrupt, a greater portion of the marginal rays could be 

 collected, and the aperture of an " eighth " was at once brought 

 up to 130° or more. 



At this time the author had been making some experiments 

 Phil. Mag. S. 4. Vol. 45. No. 299. March 1873. Q 



