96 



THE AMERICAN MONTHLY 



[May, 



As regards this matter it may also 

 be said that it is not usual for investi- 

 gators in this field — at least so far as 

 we are aware — to use oblique light 

 for illumination. The Abbe con- 

 denser is used very largely, but prob- 

 ably it finds its greatest application 

 in the study of sections of tissue, in 

 which the organisms are distributed 

 and stained. A flood of light from 

 the condenser thrown upon such a 

 preparation makes the tissues almost 

 invisible, and causes the deeply col- 

 ored bacteria to stand out with 

 wonderful clearness. But this is not 

 due to the angle of the objective. 

 The condenser is useful in this work 

 moi'e on account of the control it 

 gives over the light than for the an- 

 gular illumination it is capable of giv- 

 ing. If we are in error concerning 

 this matter we would be glad to have 

 some more practical observer correct 

 us. 



As regards our own experience, we 

 have yet to see anything more of bac- 

 teria with a homogeneous-immersion 

 lens than with a Jj by Spencer, glyce- 

 rin-immersion. What few observa- 

 tions we have made have mostly been 

 conducted on mounted specimens, and 

 the Spencer jL was carefully com- 

 pared with a Zeiss J^, without any 

 noticeable difference. Yet on Am- 

 phipleura the Zeiss is far superior to 

 the other. 



So much for the ordinary study of 

 the bacterial organisms. From this 

 we may pass to a very brief notice of 

 another kind of observation, requiring 

 the greatest skill of the observer and 

 objectives of the highest excellence. 

 Dr. W. H. Dallinger stands almost 

 alone in the thorough study of the 

 life-history of certain monads. His 

 opinions, therefore, are of great 

 weight concerning the best objectives 

 for such work. In his address as 

 President of the R. M. S. of London, 

 published in the April number of the 

 Journ. JR. M. S., he states that for 

 continuous observation of monads 

 he uses only dry lenses, a j^ ^ ^ ^ ^ 

 and a -^ being the favorite ones, the 



-^ being chiefly used. He then says : 

 ' But beyond the work of continuous 

 watching, when the opportunity pre- 

 sents itself, there is the work of de- 

 velopmental moi-phology, of discover- 

 ing all the details of the adult form, 

 and of thoroughly demonstrating the 

 changes that ensue in the completion 

 of the life-cycle. It is here that first, 

 water immersion, and now, above 

 all, homogeneous lenses have been to 

 me of untold value.' He has used in 

 his investigations a ^^ n. a. 1.47, a 

 -^j N. A. 1.38, a gi(j- N. A. 1.38 and a 

 1^ N. A. 1.50, all by Powell & Lea- 

 land. Of all these we should infer 

 that the | is regarded by Mr. Dallin- 

 ger as superior to the others. This 

 work of Dr. Dallinger is of the most 

 refined and delicate character, involv- 

 ing the detection of the extremely 

 delicate long flagella with which the 

 organisms are provided, as well as 

 the study of the structure of their 

 bodies. 



We have presented this matter for 

 the consideration of those who may 

 be interested, and in conclusion we 

 can only say that the weight of evi- 

 dence from practical observers indi- 

 cates that there are advantages in the 

 use of homogeneous-immersion objec- 

 tives for the study of bacteria ; but 

 whether these are due to the wide 

 angular aperture or to excellence of 

 construction we are not prepared to 

 say. 



o 



Electric Illumination. — There 

 are continually coming into notice 

 new devices in lamps and batteries 

 for electric illumination for medical 

 and microscopical purposes. Con- 

 siderable advance has already been 

 made in the matter of electric illumi- 

 nation applied to the microscope, as 

 the pages of this Journal in the past 

 have shown. But it must be con- 

 fessed, much still remains to be done 

 before one can sit down to a micro- 

 scope in his home, and turn on the 

 electric light like gas-light. Perhaps 

 it is too much to expect quite so great 

 convenience, yet if it is to be brought 



