18 
On the Limits of the Optical 
scope. The opinions expressed by experts and authorities on 
definition, penetration, resolution, aperture, &c., as being so many 
separate powers or qualities, besides savouring strongly of a mytho- 
logical period in the history of the microscope, have only retarded 
the search in the right direction, viz. by physical analysis and 
physiological study of optical phenomena for true causes of the 
effects observed. And in fine it must be confessed that our hand- 
books fail greatly in respect to theories of the microscope, however 
valuable their information on practical and mechanical subjects, and 
more especially on all branches of science involving skilful me of 
the instrument. 
In the absence of such handbooks as the German students 
possess, and of which the work of Nageli and Schwendener might 
be cited with admiration as an example, the scattered articles and 
shorter notices in our serials rise into comparative importance. 
But it will scarcely be contended that such desultory and dis- 
connected communications and such remarkable disputes respecting 
easily determined facts, should be accepted as an equivalent of the 
systematic theory and practical demonstration which distinguish 
foreign study of optics applied to the microscope, from our yet 
unlearnt, or at least unwritten, micrographic science. 
Various communications bearing more or less on the optical 
capacity of lens systems constructed on given formulae or for em- 
ployment as "dry" or "immersion" objectives, have appeared 
in the ' Monthly Microscopical Journal,' the * Quarterly Journal 
of Microscopical Science,' and the * Transactions of the Koyal 
Society ' during present and preceding years. Of these, one series 
of papers published by Dr. E. Pigott claims to be a mathematical 
exposition of optical laws governing the divergence and dispersion 
of rays of light transmitted through different kinds of glass. 
Another series of papers by Mr. Wenham takes the practical 
direction to which English microscopists mostly incline. The 
communications of Mr. Sorby have enriched microscopic science 
with the most ingenious and successful applications of spectrum 
analysis that any country can boast. To all these gentlemen 
the English student may feel equally indebted for their respective 
labours. And the mention of these in juxtaposition with the 
work of so great an authority as Professor Helmholtz and so con- 
scientious a workman as Professor Abbe, is not only due as a re- 
cognition of the individual services, but also as a proof of the 
higher direction of study now being pursued in England by 
amateur microscopists. As a humble member of this numerous 
class, the present writer ventures to refer to the early date of 
Mr. Wenham's communications when he stood almost alone as 
the pioneer of a future micrographic science, and to bear thankful 
testimony to the practical experience and sterling value of all 
