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IV. — Object-glasses and their Definition. 



By F. H. Wenham, Vice-President E.M.S. 



Under this title I have no wish to trespass on the patience of the 

 readers of this Journal. I must, however, remind microscopists 

 that until quite recently they have been kept in practical ignorance 

 of the tools they have been working with. Much valuable in- 

 formation has been hitherto exclusively in the hands of the principal 

 makers of object-glasses, who years ago, under an impKed con- 

 fidence that I had no immediate intention of divulging any point 

 that they held as a secret, did not hesitate at a mutual exchange of 

 information. This time has passed away, and the number of 

 makers that have since appeared render it no longer possible or 

 desirable that such knowledge should be confined to the few, but, 

 in order to extend the march of improvement, that the question of 

 object-glasses should now be thoroughly discussed in all its optical 

 conditions and practical bearings. 



From its recent controversial character, some of the information 

 has been adduced in rather a desultory manner ; at present this 

 can scarcely be avoided, as a few of the questions mooted still 

 remain unanswered. The first relates to the Podura. The return 

 of temperate weather has brought out the insects, and I have now 

 no dijfficulty in finding various specimens. I have torrefied some 

 of the scales for the purpose of gaining some further evidence of 

 their structure. The method is as follows : the insect is placed on 

 a piece of velvet, and the thin-glass cover laid on its back. In its 

 struggles to escape, a quantity of scales will be detached and adhere 

 to the glass. This is then laid (scales uppermost) on the end of a 

 piece of thin bright steel (an old worn table-knife will answer well), 

 and held over a candle till the steel turns to a very dark blue or 

 purple. The scales will at this heat be completely scorched. Of 

 course every time the operation is repeated the kni'fe must be re- 

 pohshed, in order again to see the right temperature by colour. 

 After this treatment the scales in a great measure lose their trans- 

 parency, so that we are far less hable to form erroneous opinions of 

 structure arising from false refractive figures. 



Viewed by the dark field illumination, as obtained by the 

 parabohc condenser, they no longer appear of the characteristic 

 blue colour, but are of a reddish-brown tint. The markings are 

 most decidedly plainer as seen thus opaquely. Most of the scales 

 have become more or less distorted and curled, like parings of horn 

 or dried-up leaves. Both at the longitudinal bendings and inclined 

 sides most satisfactory perspective and profile views may be obtained 

 of the ribbings or markings, aU of which, on both sides of the 

 scale, run longitudinally without any oblique crossing, and still 



