ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
667 
normally and pathologically, has been made a splendid possibility. The 
“ laying ” of optical “ ghosts,” the elision of complicated and confusing 
foci, by beautiful optical construction, is of incalculable value. It gives 
certainty and precision to all work done. 
But we must be careful now not to reintroduce the ghostly element 
by false interpretation. I am increasingly convinced of the possible 
danger of employing shafts of oblique light only in one azimuth. The 
peril of misinterpretation is enormous. 
Indeed, I have a growing conviction that all small cones of illumina- 
tion may be fraught with danger, at least to the amateur. 
Our German fellow-workers have only lately risen to the perception 
that the condenser is of value at all, but the condenser they universally 
employ is chromatic. Its aberration is enormous. True, their greatest 
microscopical optician has within the last three years seen the value of 
achromatism, and has made an achromatic condenser ; and its value, 
as compared with that of the chromatic combination, is inestimable. 
But surely if we are to get the purest results from apochromatized 
lenses, if we are to get a focal image absolutely freed from ghostly con- 
fusion, we should have an apochromatized condenser, and a condenser 
of the greatest possible numerical aperture. 
In England those who have made microscopy a special pursuit, have 
long worked with fine achromatic condensers. I am glad now to know 
that the first apochromatic condenser yet made has been produced by the 
firm of Powell and Lealand, and it only needs an hour’s trial in expert 
hands, and experienced judgment, to discover its great superiority. It 
has an aplanatic focus of * 9, and even if oblique beams only in one 
azimuth be used, their danger is reduced to its lowest. But it is by the 
employment of large cones of illumination, and not with small ones, that 
I say cautiously, but still with emphasis, the finest and truest results are 
to be obtained. 
We may well pause before we finally pronounce on this subject, but 
it certainly is one that must be settled in practice, however present 
theory may point ; and we must all feel that the remarkable paper of 
my friend Mr. Nelson on this subject, read to us during the past year, 
must be gravely considered, and made a starting point for patient 
research. For it is by such means that an amateur club like ours may 
contribute what is of permanent value to the professors and students who 
use the Microscope so largely in our schools of biology and medicine. 
But, nevertheless, it is possible to push one phase of optical con- 
struction so far as to accomplish the object, but to leave doubtful the 
usefulness of the object gained. 
We have all heard of the new objective produced by the firm of Zeiss, 
of Jena. It has a numerical aperture of 1*60. This from one point of 
view, is a great advantage. None would have greater reason to hail 
it than I, in the special work with which my life has been largely 
occupied. 
Now, I have spent five consecutive days in the close and critical 
examination of one of these objectives, which, so far as I know, has been 
in no other hands but my own and those to whom I have shown it. I 
desire to take the sole responsibility of estimating its value. In my 
hands it is an extremely beautiful lens; it is well centered, well corrected, 
