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 NOTES AND MEMORANDA. 



Information required as to Microscopic Powers. — A gentleman of 

 Melboui'ne, Australia, who signs himself H. H. has written a letter to 

 ' Nature,' of June 5, making some sensible inquiries on the subject of 

 microscopic powers. We think the letter so likely to interest certain 

 of our readers that we reproduce it in full. The writer says : — I am 

 following up some investigations and experiments in which I require 

 certain data, which, however, I cannot at present arrive at, not being 

 in possession of sufficiently delicate and exact instrumental appliances. 

 The information which I now desire to elicit from some more ex- 

 perienced observers than myself is of such importance as to be both 

 useful and interesting to many of youi- readers, and I therefore crave 

 your insertion of this communication. The information I require is 

 all the more important as having a bearing uj)on many questions which 

 are now attracting public attention, such as spontaneous generation, 

 the initial stage and transitional forms of living organisms, also various 

 researches in experimental physics, chemistry, &c. I desire to arrive 

 at the following data : — 



1. What is the estimated dimensions of most minute particles of 

 matter which can be visible, under any circumstances or conditions, 

 under the highest powers of the microscope ? I leave out of considera- 

 tion (under this head) the question whether such matter is living or 

 dead, organic or inorganic, or in fact regardless of any of its properties 

 whatever except its mere visibility as a minute portion of matter. 

 Some observers speak of visible particles ^-qoWo*'^ ^^^ iJooVoo^l^ of an 

 inch diameter ; this is surely near the limit. 



2. What is the best or most accurate method of arriving at an 

 estimate of the dimensions of such minute objects as are too small to 

 admit of actual measurement by any of the ajipliances now in use? 

 Every microscopist knows from experience that objects may be dis- 

 tinctly visible, not as a mere point, but having an appreciable dia- 

 meter, and yet be too minute for actual measurement to any degree of 

 accuracy. 



3. Have the most recently constructed microscopic objectives, such 

 as the gioth or sVth, any advantages over the y'^th or y^^th inch objec- 

 tives in the determination of the data above referred to ? and have 

 immersion lenses any advantage in this respect ? I find some difference 

 of opinion on this point. Some microscopists consider that a really 

 first-class ^V*^ ^i^h the use of deep eye-pieces will enable us to see 

 anything whatever which can be seen by any other objective of shorter 

 focus. On the other hand, it is evident that a great number of the 

 most experienced microscopists think otherwise ; and from the very 

 fact of their purchase of such expensive high powers, argue that such 

 lenses are found to supply what other powers cannot accomplish. It 

 appears to me that there is too much of vague and indefinite assertion 

 in regard to the comparative powers and qualities of microscopic objec- 

 tives, and it is very desirable that some more definite results should be 



