266 DR. ROYSTON-PIGOTT. 



axis at a point distinct from B. The great bulk of the rays, 

 except the red or reddish yellow, converge accurately to b, 

 whilst the residuary rays of reddish yellow converge to R, so 

 that we either in general get achromatism with residuary 

 spherical aberration, or reddish-yellow rays when the sphe- 

 rical aberration is destroyed. In other words, the achromatic 

 focus and the aplanatic are not identical. 



Spherical aberration, as detected by the methods I have 

 recommended, displays itself in a colourless milkiness or 

 whitish smoke-like cloudiness. Dissipate this by better 

 spherical corrections, and it is immediately replaced by all 

 brilliant points becoming irradiated with the orange-red halo. 



The obstinacy with which the false Podura markings are 

 adhered to in correcting glasses almost necessitates a wrongly 

 corrected spherical aberration. These markings cannot be 

 seen except the upper beads be focussed through. 



If the orange-red rays be absorbed by the ammonio-sul- 

 phate-of-copper solution transmitting monochromatic light, 

 definition actinically is much more easily produced. But the 

 eye receiving ordinary compound light, whilst the spherical 

 aberration is neglected to the advantage of the achromatism, 

 cannot possibly see as well as the actinic monochromatic rays 

 potentially define and depict. A point may be ascertained 

 in the axis where the aberration may be almost extinguished, 

 when it is no longer confused with uncorrected red and 

 yellow rays. In other words, the blue rays may be brought 

 accurately to the same mathematical point in the axis, whilst 

 all others would vary more or less or aberrate. 



The mathematical conditions of the least possible colour 

 are improved by these changes : — 



1. Varieties of position of the lenses. 



2. Varieties of refractive indices in the glasses employed. 

 S. Varieties in actual focal lengths. 



4. Varieties in the immersion fluids. 



5. Variety in the nature and thickness of the glass cover. 

 In many cases more delicate adjustments can be made by 



changing the distance between the back or larger combina- 

 tions of the objective. The combinations for producing the 

 best glasses are endless, like those of an extensive peal of bells. 

 In the colour test (described by the writer in a paper sent 

 to the Royal Microscopical Society, May 21, 1869, page 

 302, December number of 'Journal'), the phenomenon of 

 the very finest definition obtainable by the very best objec- 

 tives then made, presented to me very considerable difficul- 

 ties for explanation, which I was not then able to grapple 

 with. More recent experiments, carried on with objects 



