10 
In reply to numerous enquiries and. in some cases, publicly expressed 
doubts respecting the 
durability of apochroinatic lenses 
we append the following statements. 
All glasses used in the construction of apochromatic lenses have been 
amply tested through several years' experience and offer a fair guarantee for their 
immutability, at any rate in temperate climates and on condition that the lenses 
are not subjected to any undue treatment. This statement does, however, not 
apply for the present to suit hot climates where an exceptionally damp and hot 
atmosphere constitutes an abnormally active decomposing agent, which even the 
older (silicate) glasses do not permanently resist; in these cases we disadvise 
the use of apochromatic lenses. A few isolated cases have indeed occurred even 
in our climate in which the one or the other of the constituent lenses of certain 
apochromatic systems have become turbid; but it would not be fair to hold us 
responsible for such contingencies. For be it remembered, the introduction of 
apochromatic lenses by us necessitated the application of entirely new materials 
which until then had not been used or tested. The first lenses, necessarily, 
lacked the kindly aid of experience and it is not surprising that a few failures 
should have occurred. Whereever varieties of glass used in the construction of 
lenses proved to be liable to deterioration, the lenses containing such glasses 
were subjected to reconstruction and all doubtful glasses supplanted by such 
that were known by experience to possess greater durability. Even as now 
constructed the apochromatic lenses are sensitive with respect to moist stagnating 
air and in damp hot climates ; it is, therefore, advisable to keep the lenses, when 
not in use, in air-tight receptacles. 
As in former cases we shall also in future in any such cases, where one 
of the lenses of an objective supplied by us should exhibit spontaneous changes, 
not hesitate to repair the damage at our cost. To this promise we attacli however 
the condition that the objective in question be sent to us previous to any 
attempt being made in this direction by others. In no case should 
such an objective be disconnected, with a view to clean it, by any other but an 
expert, as this might suffice to convert a slight irregularity, that could be easily 
corrected by us, into a serious or even irreparable defect. 
