THE AMERICAN MONTHLY 



[January, 



venient focal length to be employed 

 for the purpose. I have now to 

 report briefly the results of my ex- 

 amination of three amplifiers, by 

 Zeiss, sent me by Professor Abbe, 

 with the request that I would test 

 them photographically. Each of 

 these amplifiers had a virtual focus 

 of about 10.5 centimeters, and were 

 achromatic, but differed in con- 

 struction ; one was a meniscus, one 

 a plano-concave, and one a bi-con- 

 cave combination. Each was 

 mounted with a double screw, so 

 that either side could be turned to- 

 wards the objective. Professor 

 Abbe explained to me that the first 

 was the form which Zeiss has been 

 making for use at the end of the 

 draw-tube, but added that he has 

 "no reason at all for considering 

 this the best form, as this, of course, 

 could be made out by trial only." 

 He requested me, therefore, to 

 make the trials to ascertain " which 

 of the three would give the best 

 performance, and in which po- 

 sition." 



I have now made these trials, and 

 communicated the results to Pro- 

 fessor Abbe ; but, as the subject is 

 one of general interest, I have 

 thought it worth while to publish 

 this brief memorandum. After 

 having ascertained the best position 

 which each of the new amplifiers 

 should occupy, in order to give an 

 image of Amphipleura pellucida of 

 a convenient size, I began by mak- 

 ing a photograph of a pair of frus- 

 tules of this diatom, with the Zeiss 

 one-twelfth inch and an amplifier 

 by Tolles. I then made a series of 

 photographs with the same object- 

 ive, all the other conditions remain- 

 ing unchanged, except that I short- 

 ened the distance enough to obtain 

 approximately the same magnifying 

 power, and substituted successively 

 the three amplifiers of Zeiss, which 

 I used first with one side, then 



with the other, turned toward the 

 objective. The result of the trial 

 was, that with no one of the 

 Zeiss amplifiers, in either position, 

 could I obtain as good results as 

 with the Tolles amplifier ; the 

 images they gave were somewhat 

 deficient in brilliancy of definition, 

 but they were still more wanting in 

 flatness of field. As between the 

 several Zeiss amplifiers, I am dis- 

 posed to give some preference to 

 the meniscus, but the difference is 

 not very conspicuous. I see no 

 reason why it should be impossible 

 to construct amplifiers of this focal 

 length which should give flat fields ; 

 yet, certainly, the difficulty appears 

 to be greater than with those of 

 longer focus, and I have requested 

 Professor Abbe to ask Zeiss to make 

 for me an achromatic meniscus of 

 the focal length T have been suc- 

 cessfully using, and will report on 

 its performance so soon after I re- 

 ceive it, as I can give it adequate 

 photographic trial. 



Notes on Fresh-Water Algae. 



These notes are designed to 

 assist the learner, as he begins the 

 study of the algse, not by discus- 

 sing classification, or in any parti- 

 cularly scientific or systematic man- 

 ner, but by affording brief, gene- 

 ral descriptions of the plants found 

 in our ponds and ditches, accom- 

 panied by one, or more, illustra- 

 tions in each number of the Jour- 

 nal. The practical value of such 

 a course will, it is thought, be very 

 great ; not only as an aid to the 

 beginner, but in the course of time 

 a good series of illustrations of 

 Algse will be accessible for refer- 

 ence, which will be more valua- 

 ble than the best descriptions to 

 the advanced student. 



In the preparation of these notes 

 the editor is assisted by Mr. Francis 



