160 A New Formula fur a Microscope Object-glass. 



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 diameter : 

 dense flint would have remedied this to some extent ; but its 

 liability 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 

 rendered 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 in 

 the construction of an object-glass in the form of Fig. 2. Mr. 

 Lister having favoured his " eighth " with an examination, was 

 good enough to communicate his late improvement of the triple 

 back. No time was lost in giving this a trial, the result of which 

 proved that excessive negative aberration or over-correction could 

 readily be commanded with lenses of shallow contact-curves. During 

 these trials all chromatic correction was obtained by alterations 

 in the triple back ; for it was found that the colour-correction 

 could not be controlled by a change in the concave surface of the 

 triple front, as the negative power of the flint here appeared to be 

 feeble, requiring a great difference in radius to give a trifling 

 result. For this reason the front concaves were formed of very 

 dense and highly dispersive flint ; the cause of this was analyzed 

 by a large diagram, with the passage of the rays projected through 

 the combination, starting from the longest conjugate focus at the 

 back. This proved that the rays from that focus passed through 

 the concave flint of the front nearly as a radius from its centre, or in 

 such a direction that its negative influence was almost neutralized. 

 It is well known that a lens may be achromatic for parallel rays, and 

 under-corrected for divergent ones. The utmost extent of this con- 

 dition was apparent in the object-glass under consideration. 



This led the author to the idea of the single front lens of crown 

 glass, which gave a fine result at the first attempt, as the back 

 combinations' to which it was applied happened to have a suitable 

 excess of negative or over-correction existing in the triple back 

 alone, the middle being neutral or nearly achromatic. Still there 

 was a defect remaining as positive spherical aberration ; and this 

 was afterwards cured by giving additional thickness to the front 



