CORRESPONDENCE. 
51 
of a system. But for the practical microscopist (meaning especially 
the histologist) they have many faults which restrict their use. For 
instance, in all the three powerful systems the distance from the object 
is exceedingly small, and they demand therefore far thinner cover- 
glasses than the equally powerful systems of Hartnack, Zeiss, and 
Eelthle. Besides, the picture obtained with the usual mirror illumi- 
nation is, scientifically speaking, of less use. Placing, however, the 
achromatic condensing lens between mirror and object, and regu- 
lating its position carefully, the pictures of the organic objects 
assume a totally different appearance, with clear outlines and without 
coloured edges. (To this refers my second epistolary remark follow- 
ing an equally epistolary reproof on the subject.) Finally are these 
glasses very deficient as regards 'light,' and require it direct from 
the sun (this was specially mentioned in Hasert's letter after my 
letter finding fault with it), which is a great obstacle to their use. 
Most decidedly, too, I must blame the obnoxious yellowish colour of 
the ' fields of vision,' which renders the use of them distasteful." 
These words, I think, are straightforward and denote " fair play," 
and are a proof that praise and censure are bestowed where merited, 
and exactly in the same place M. Hasert will find them in my letters. 
I dare say it may interest your readers to inquire how many of our 
noted histologists are using Hasert's objectives. Finally, as regards 
the " neatly told story " of M. Hasert of the occurrence at the 
meeting of the naturalists.* The following remark on the above- 
cited page will, I think, deal with it effectually, for it says : " ' I for 
my part cannot unconditionally ' agree with the praise lately (' Botan. 
Zeitg.' 1863, No. 10) bestowed by Professor Hofmeister on the instru- 
ments by Hasert." The " why " is easily explained by the faults just 
censured ; besides, I think it my duty to advise people not to be led 
implicitly by the stylish advertisements of Prof. Hasert ; for instance, 
page 124, 'Botan. Zeitg.' 1864, says: "The ' correction 'for the different 
thicknesses of the covering glass is no longer required ; trials with single^ 
double, or triple thickness of the glass yield equally good results." When 
I, however, at the meeting in Giessen (autumn, 1864) sought by means 
of a " Pine section " to try such an objective system insensible to 
thichiess of covering glass, I found that the latter, under 0 • 1 mm.,| 
although perfectly of avail for the most powerful systems of Hartnack 
10 (yi^ inch engl) and 11 {-^^ inch engl) became useless, 
I hardly know for what purposes any practical microscopist could 
use such a system." 
I believe the above is sufficient to prove that M. Hasert's state- 
* How, for Instance, does this good gentleman know that the covering glass 
of my preparation measured at least 0-75 mm., as he had neither measured it, 
nor could do so at the time ? I suppose I need not tell you that I don't use for 
such objects a covering glass of looking-glass materials. 
t Exact thickness I measured 0*08 mm. The preparation is up to this day 
in my possession, and has ever since done its duty with the systems of Gundlach 
IX., 1868 ; Seibert IX., 1873 ; Beneche XII., 1869,— all three equal to inch 
engl. Zeiss' immersion No. 3 = inch engl, without the glass cover "having 
been found too thick. 
