A MUSEUM enemy: DUST— "WAITE. 



97 



Such a perfected case may be aptly compared to a piano, the 

 back of which is covered, for acoustic jDurposes, 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 sutiicient "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 eflicacy. The test cases were supplied 

 with white cards, whereon were j^laced 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 sufiicient 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 



