132 NEW BOOKS, WITH SHORT NOTICES. 



quality of the lower objectives rather than deepening focal length. 

 Observers are more numerous every year who prefer the ^th to the 



^Vth and ^Vti^,-" 



" Appendix. 



" The law of displacement followed by the final focal image corre- 

 sponding to a minute displacement of the internal lenses of a complex 

 objective, the front lenses or facet remaining fixed, possesses some 

 interest and may thus be expressed : — 



" Let F be the distance of the final focal image when the objective 

 lenses are closed together. 



" F _j_ ^ F its distance when the front sets of the objective are dis- 

 placed by a quantity d x. 



" Then it will be found if /i be the distance of the virtual image 

 conjugate with the object as formed by the front set of lenses, 



^Y -Ax:: -F^:f,^; 



and consequently every sli'ght change of the screw-collar of an adjust- 

 ing objective produces comparatively a very large displacement in the 

 final focal image, and therefore of the traversing image searcher ; so 

 that the searcher-traverse represents a movement conjugate with the 

 objective index. Again, since this traverse towards the objective 

 encounters rays of increasing divergence, an increasing breadth of 

 pencil is encountered by the lenses of the searcher, and its own pecu- 

 liar aberration receives an instantaneous increase, which introduces an 

 important new element in definition ; it having been observed that the 

 glasses must be very gradually over-corrected as the image is formed 

 nearer the objective." 



The rage for diatom resolving microscopes requiring an enormous 

 angular aperture will now probably be abated in favour of instruments 

 long preferred by working naturalists, to whom a just display of an 

 object in relation to its parts, by penetration or depth of focal per- 

 spective, with a working distance between the object and objective 

 sufficiently roomy to admit manijiulation, are conditions indispensable 

 to organic structural research. We cannot but think Dr. Eoyston- 

 Pigott's invention a valuable one, even if it does nothing more than 

 substitute low eye-pieces for deep ones, and enable the half-inch power 

 to work at a greater distance, with a power of 800. We observe with 

 pleasure the paper was communicated by Professor Stokes, whose name 

 has so long been associated with advanced optical science. 



We think it should be here intimated that Dr. Eoyston-Pigott 

 has tried a variety of convex and concave lenses for searchers. The 

 " Barlow " achromatic concave lens, when not made too deep, produces 

 some good effects, gives an inverted image instead of erect, but also 

 often displays a " central ghost," and is incapable of producing the 

 automatic and differential adjustments so much jn-ized in transcen- 

 dental definition. There will doubtless be many cheap imitations, 

 which, it may be feared, will only caricature the properly-constructed 

 instrument, which must be regarded as a new aid to microscopical 

 research. 



