798 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Tho rational gradation in tlio focal lengths of the apocliromatic 

 objectives has made it possible to essentially simi)lify both in calculation 

 and tabulation the measurements to bo made with them. Tho micro- 

 motor eye-piece (fig. 133) used is a compensation eye-piece, No. 6, of tho 

 usual form (new constrnction), and a graduation 

 Pjq 2;^3 in which the intervals for an ideal objective of 



1-0 mm, focal length (with normal tube-length) 

 are O'OOl mm. = 1 /x. 



The value of an interval rises in tho same 

 ratio as tho focal lengths of the objectives, and 

 is represented by tho same numbers, it is there- 

 fore 



2*0 /A for apochromatic 2-0 mm.< i .Vn m a ^ 



2-5 „ „ 2*o „ 



o.o o.A J (1*30 and 



^ - >. » "^ " " t 1-40 N.A.) 



4-0 „ „ 4-0 „ 



8-0 „ „ 8-0 „ 



16-0 „ „ 16-0 „ 



so that the same number denotes the interval in 

 terms of fx and tho focal length in mm. The use of this eye-piece there- 

 fore renders a special table unnecessary. 



Measurements made in this way will always be correct within a 

 slight percentage, since individual variations of particular eye-pieces 

 and objectives always lie within very small limits. If, however, it is 

 necessary in special cases to find a very exact value of an interval for a 

 particular objective, it must be tested in tho ordinary way by a etago 

 micrometer, and then the small deviation in the value of an interval 

 from its true value for a given objective, as expressed by its number, can 

 bo corrected by a slight alteration of the tube-length. In such a case 

 the objective in question is focused upon a stage micrometer, and if 

 an interval of the micron-division does not cover exactly so many 

 thousandths of a mm. as are given by the focal length of the objective, 

 the correction is made by a small lengthening or shortening of the tube- 

 length, and the exact tube-length shown by Ihe graduations of tho draw- 

 tube noted for each objective. 



American v. Foreign Microscopes ; the Verdict of an Impartial Expert. 



[Results of Dr. H. J. Detmers' examination of objectives by Lcitz, Seibort, and 

 Zeiss.] St. Louis Med. and Surg. Journ., LV. (1888) pp. lGO-3. 



(3) Illuminatingr and other Apparatus. 



Eternod's Drawing-board.* — Prof. A. Etcrnod recommends tho uso 

 of a drawing-board invented by him, and which he has found useful for 

 microscopical drawing, as it is very stable and easy of management. It 

 consists of a shallow box (fig. 134, a), the sides of which are strongly 



* IntL-rnat. Monatscbr. f. Anat. u. Iliatol, ii. (1885) pp. 269-70 (G figs, of a 

 plate). 



