CORKESPONDENCE. 29 



Powell and Lealand required a full statement of the facts with regard 

 to siicli of tlieir objectives as I have measured. 



The Surgeon-General's Office possesses a number of objectives by 

 the leading American, English, and Continental makers. Among these 

 are the following by Messrs. Powell and Lealand. One -^g*^ ^^^ one 

 ^*^th, both dry lenses. Two |ths and two yy^^^^' ^f'-ch furnished with 

 two fronts^ one for use wet and one dry. One of the ^ths was origi- 

 nally made for dry work only, and the wet front subsequently added by 

 the makers ; the other y ^ ^^^ ^^^ ^^^ *'^o i^^^ were furnished in the first 

 instance with two fronts. The following table gives the magnifying 

 power of each of these objectives without eye-piece at four feet (48 

 English inches) distance from micrometer screen. The micrometer 

 used was a broken Nobert's plate mounted with the lines uppermost, 

 and the image of the micrometer was projected on the screen by direct 

 sunlight. 



Magnifying power'MagnifyJngpow^^ 



whoii nT,r.,^..oroH I ^' 1"" COUtCllOD 



Objective. 1 when uncovered 



for thickest cov er 

 same distance. 



Powell and Lealand's ^th dry j 2200 j 3100 



„ „ J-jth , ' 1250 1425 



„ „ dry Ji^th (No. 1) .. .. , 790 ' 930 



New immersion front to ditto 975 | 1180 



Powell and Lealand's immersion yLth (No. 2) .. 900 1100 



Dry front to ditto 770 910 



Powell and Lealand's immersion Ath (No. 1) . . 450 500 



Dry front to ditto 425 490 



Powell and Lealand's immersion ^th (No. 2) . . I 460 I 505 



Dry front to ditto 1 450 | 510 



i 



The above determinations were made with the utmost care, each 

 observation being several times repeated. No eye-pieces or amplifiers 

 were used in order that the conditions might be as simple as possible, 

 and a long distance was taken, that small unavoidable errors in its 

 measurement might have as little influence as possible on the results. 

 The magnifying powers were obtained by dividing the dimensions of 

 the part of the micrometer selected into the dimensions of the image, 

 carefully measured on the screen, and the results therefore fairly re- 

 present the performance of the objectives named under the conditions 

 stated. 



How are these results to be interpreted, and how ought the ob- 

 jectives in question to be named? Mr. Charles Stodder tells us* that 

 " objectives are named when adjusted for uncovered objects, a fact not 

 generally known by purchasers. The power increases, i. e. the focus 

 is shorter as the collar is turned to work through the covering glass." 

 But how in the case of a single objective with wet and di-y front, or 

 one which by mere change of the screw-collar works from dry to 

 wet ? I believe that the commercial practice in this case is to name 

 the objective by its performance at uncovered dry, and this practice 



* This Journal, October, 1871, p. 203, notr. 



