A MUSEUM EXEMY : DUST— WAITE. 
97 
Such a perfected case may be aptly compared to a piano, the 
back of which is covered, for acoustic purposes, with a textile 
fabric ; this has also the secondary and unintentional property of 
relieving the pressure of air, and guarding the interior of the 
instrument from the intrusion of dust. There can be little doubt 
that the efficacy of the filter depends as much upon the flexibility 
of the material employed, as upon its filtering properties. 
In order to put the theory to a practical test, two precisely 
similar cases were constructed and placed at my disposal. After 
the joints had been carefully closed, one was fitted with a filter 
of cotton- demette, and the other with a diaphragm of oiled silk 
placed on loosely so that sufficient “slack” or “bag” was allowed. 
Previous experience had shown that when a filtering material 
was used, either time or extreme conditions of dust would be 
required for testing its efficacy. The test cases were supplied 
with white cards, whereon were placed coins, glass slips and 
objects designed to register any dust which might be deposited. 
They were screwed up in August 1894, and placed in the central 
fish and reptile gallery. 
Early in 1895 it was discovered that the roof of this gallery 
was infested with “white ant” to such an extent that imperative 
repairs were necessary. This occasioned extreme conditions of 
dust, and it is not too much to say that the dust created during 
the removal of the plaster and rotten wood, which process occupied 
several weeks, was greater than would ordinarily have been 
formed in many years. The specimens in the two large bird 
galleries adjoining, which had been screened oft, had to be 
thoroughly cleaned and replaced before the galleries could be 
reopened, so thickly were they covered with dust. In the light 
of subsequent events I venture to say, that had the cases been 
provided with flexible diaphragms, this would not have been 
necessary. 
On opening the test cases (November 1895) in the presence of 
several of my colleagues, the results were even more conclusive 
than had been anticipated. Considering the ordeal through which 
it had passed, the filter had acted well ; the dust deposited was 
very fine, but sufficient in quantity to show in how far it had 
failed. To finger one’s name on the white card on the floor of 
the case was an easy matter, but the result was more apparent 
when the coins were lifted, When magnified, a glass slip 
resembled, to a non-astronomic eye, a photographic negative of 
the Milky Way. 
On the other case, that is, the one provided with the oiled silk 
diaphragm, being opened, no trace of dust whatever could be 
discovered, and when placed beneath the microscope, a glass slip 
