50 
CORRESPONDENCE. 
and at his instance I have more specially examined them with the aid 
of my strongest ' objective systems,' and fully convinced myself of the 
correctness of his views. An unmistakable position has thus been 
gained as to how the diagonal strias may be seen with good instru- 
ments." 
Now although the above words do contain an indirect praise of 
Hasert's systems of 1861, it is surely not averred that they are the 
best, or unsurpassed for every investigation, neither is it said that my 
own examinations were made with the aid of Ms glasses, which in fact 
they were not ; at the most it was shown that " Hasert's objectives " 
in question were, in a certain sense, superior to the older ones with a 
smaller angle of aperture. 
On page 143, treating of the colouring of the " field of vision," it is 
said, " I have always found the yellowish colouring to be most strongly 
developed in various gradations, and it becomes, for instance, with 
some of Hasert's objective systems (here produced by the glass, or rather 
the liquid enclosed in the lower lens) so strong, that it makes the 
whole field of vision resemble a sheet of white paper covered with a 
thin layer of gum, and positively becomes troublesome to the eye, and 
obscures some more delicate shadings, ' and I Jiold this prominent colour 
to he a most serious fault of any objective system, rendering it almost 
useless for the practical microscopist, even if in other respects it answers 
well.' " 
This expresses, no doubt, most decided censure, but every micro- 
scopist who ever had objectives of Hasert's of that period will, I feel 
sure, agree that it is deserved (vide opinion of H. von Mohl and 
Professor Schacht). 
On page 169 it is said, with reference to Hasert's strongest objective 
inch, " On Nobert's plate are solved the 18th, and if illumed 
obliquely the 26th group." (The discrepancy between the results as 
published by me and contained in my letter, is explained by the fact 
of my having used the clearest white cloud light (" hellstem weissen 
Wolkenlichte ") at the first trial ; whilst the comparative trial after- 
wards was made with a clouded sky and the careful shutting off of all 
side lights.) In the same manner will " central " illumination solve 
Pleurosigma angulatum, Grammatophora marina, and Navicula veneta ; 
and " oblique " illumination all natural test-objects without showing 
the same full clearness as is obtained with Hartnack's systems, 9, 10, 
and 11; and summing up then the total result of my most elaborate 
and comprehensive trials, I said here, " It follows thus, that the 
objective systems of Hasert, although mechanically most simply and 
strangely, not to say carelessly, constructed, occupy a very high rank as 
regards their solving power, and might also he said to rival the immersion 
lenses of Rartnach and Amici, hut are much inferior as regards heauty 
and clearness of the picture under central illumination. For those 
microscopists who principally study the Diatomacese (here I may be 
justified in referring to Dr. Schumann's researches) they are really 
excellent, and sure to excite the admiration of those who hold the 
solution of the Grammatophora and similar objects as the masterpiece 
