264 DR. ROYSTON-PIGOTT. 



Many high-class objectives are carelessly condemned, or 

 at least disparaged, from the following causes : 



(1) They are applied to a body which is either too long or 

 too short. 



Thus, nearly all the continental objectives, dry or immer- 

 sion, are corrected for about six inches, and even five. 

 Immense advantage will be gained by using a telescopic body 

 capable of being shortened or lengthened at will, in order to 

 search the axis for the most perfect "aristokratic image"^ which 

 the objective is capable of forming, when it will be foimd, 

 with proper screw- collar adjustments, that a new power of 

 high-class definition and amplification will be gained in many 

 unexpected cases. 



(2) Further, there is a certain thickness of covering glass 

 and a certain refractive index for it which, par excellence, 

 gives the finest efiects. 



(3) And, again, as a variation of the length of the body 

 necessarily causes also the distance of the objective from its 

 object to vary, great changes are thus made in the optical 

 conditions of vision. 



Thus, the thickness of the stratum of air in the dry lens is 

 increased as the body is shortened and the course of the rays 

 is changed, the achromatism alters, and the spherical aberra- 

 tion takes a new form ; whilst in the water lens a greater 

 thickness of water film also materially changes the definition.^ 

 On the other hand, a greater or less thicknesss of covering 

 glass causes a less or greater thickness of water film, and this 

 changes the character of the definition. In the beautiful 

 l-8th and l-16th immersion lenses now made, these eifects 

 are very evident to a careful observer. Indeed, there seems 

 to be an absohite necessity of attending to the degree of 

 thickness of the cover in producing the very finest effects of 

 which a given objective is capable of developing, the screw 

 collar being after all, as I have before stated, but a rough 

 compensation for variable thickness, good enough for the 

 tests employed at the time of its invention. 



I have not as yet been able to induce any of my friends to 

 venture upon the infallible test of objectives described by me 

 in the ' Philosophical Transactions.' The process is regarded 

 as difficult and dangerous, as bringing the observing and 

 image-forming objective into a dangerous proximity. Still, 

 . revelations are made in this way obtainable by no other method. 



* The term " aristokratic image" was introduced to denote the best 

 poicer image obtainable by given combinations of eye-pieces, searcher and 

 objective. 



'^ Messrs. Powell and Lealand fully agree with me in this view. 



