328 The Microscope. 



and looking at the bright spo^ in the back lens of the objective. 

 The difference in the performance of the objectives was certainly 

 less marked than with oblique light. The required tube lengths 

 remained about as before stated, with, however, less noticeable 

 difference in a given amount of change. I obtained a kind of a 

 glimmer of resolution on No. 19, with the apochromatic and 

 Gundlach's lenses, but nothing with any of them on 18 or 20. 

 The others were well resolved by all four objectives, Gramato- 

 phora subtilissima giving the most trouble. I have never seen 

 a balsam mounted Amphipleura resolved b}' truly central illum- 

 ination, though others have reported it with several objectives. 

 When using the condenser named, by moving it onlj' a little to 

 one side, lines c?ould be made out, but no comparative tests of 

 this kind were made. 



I had previously tried, with the help of an expert assistant, 

 the three objectives in my possession in photographing violet 

 stained bacteria with central light, showing a scared}- appreciable 

 difference, but favoring the Zeiss and Spencer over the ToUes, 

 unless the increased difficulties with the higher power proved 

 too much for the skill of the manipulatoi's. 



I have now to add a word in regard to the durability of the 

 apochromatic, the want of which has been frequently' questioned. 

 After about two years' use it became evident that this lens was 

 in some way impaired, and bj' looking through it from the back 

 with a magnifier, a haz^'-granular appearance was noticeable, 

 not due to dust on the back lens. Last March the objective was 

 sent to the makers for examination and repair. It reached me 

 again in July, as good as new, with the statement that the front 

 lens had been slightly decentred and that the repair had been 

 easily mide, and was without charge. I have no other informa- 

 tion upon this point, neither do I know what interpretation to 

 place upon the granular appearance noted. There is certainly 

 nothing of the kind visible now. 



1 



DO WASPS PRACTISE ASEPSIS? 



EDWARD GRAY, M. D. 



T IS the remarkable habit, as is well known, of certain wasps,* 

 to provide animal food for their young and to provision the 



*A11 wasps (Eiley.) 



