Analyses of Books . 
1883.] 
293 
objectives : the reason of their superiority is explained in the 
article “ Angular Aperture.” 
The distinction between water and oil immersion lenses seems 
to have escaped notice : the latter or homogeneous system, ori- 
ginally devised by Mr. J. W. Stephenson, to save the trouble and 
uncertainty involved in adjusting for the thickness of the cover- 
glass, and afterwards shown by Prof. Abbe to possess additional 
valuable properties, chiefly aperture, far beyond the limit of 
glasses working through air or water, and that precious quality 
to the working microscopist, increased working distance. 
The description of the Achromatic Condenser, good as it is, 
would have been much improved by the mention of the illumin- 
ators constructed on the immersion principle, rendered necessary 
by the employment of immersion objectives, as, if a condenser 
working through air is used with an immersion-glass, its aper- 
ture is reduced to i*o numerical aperture 180° in air. So to 
obtain the full benefit of the augmented aperture, the condenser 
must be adapted to transmit its pencil through the same medium 
as that of the objective. Such illuminators have been designed 
by Abbe, Stephenson, Powell and Lealand, and others. 
The concluding portion is occupied by practical directions for 
the use of the microscope, and carrying on observations by its 
means, and mounting and preparing objects. 
With respeCt to the Dictionary proper, a considerable extension 
is apparent when compared with the previous editions. The 
student is much assisted in finding what he requires by an ex- 
tensive series of cross-references, and those who wish to extend 
their reading will find all they require in the copious list of books 
and papers at the end of each article. A short quotation will 
afford the best illustration : — 
“ Illumination. — This has been specially alluded to in the 
Introduction (p. xxviii.), and in the articles ‘ Angular Aperture,’ 
‘ Diatomaceac,’ ‘ Polarisation,’ and ‘ Test-objeCts.’ But several 
papers have been published in recent years, describing new and 
ingenious methods of illuminating the finer and more difficult 
objects, to which we can only refer. 
“ Bibl. — Higgins, Qu. Mi. Jn., x., 150 ; Abbe, M. M. Jn., 
xiii.,77; Smith, ib., 88; Wenham, ib., 156, and Engl. 
Mechanic , 1877, 279 ; Whittell, ib ., xiv., 109 ; Bramhall, 
ib ., xvi., 102 ; Osborne, ib., xvii., 179 ; Moorehouse, ib., 
xviii., 29 ; Woodward, ib., xviii., 61 ; Edmunds, ib., xviii., 
’ 78 ; Schulze, Jn. Mic. Soc., 1878, 45.” 
The treatise “ Rocks,” by Mr. Frank Rutley, is a valuable 
addition, and is accompanied by Plate 42, containing twenty-four 
coloured figures of mineral sections viewed by polarised light. 
Much practical information is given as to the peculiar mode of 
observation required in examining mineral structures. Here the 
