646 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tnbo and Cfwi bo rotated with it. m is fixed in position by tlio ecrow a. 

 The eye-picco is movable, to suit different eyes. The illnniinator » is 

 screwed to the hohlcr of the objective h, and is turned towards the light 

 by ft small handle c. The scale should bo covered with thin glass 

 brought as near to it as possible in order that tho illuminator may not 

 bo further from the scale than is necessary. 



The author claims that his method of reading has also tho advantage 

 that errors in the dividing arc at once detected by the failure of coin- 

 cidence between the micrometer divisions and those of tho scale, and he 

 concludes with tho results of some observations with a theodolite of 

 13 '5 cm. diameter divided to one-third of a degree, which showed tho 

 mean error in an angular measurement to be ± 3", and the maximum 

 error ± 5". 



Leach, "W. — The Lantern Microscope. 



[Cf. this Journal, 1887, pp. 1019-21.] 



Trans, and Ann. licp. Manchester Micr. Soc, 1887, pp. 52-7 (1 fig.). 

 QviifN, E. P. — The Advantages and Deficiencies of the Lantern Microscope. 



Trans, and Ann. licp. Manchester Micr. Soc, 1887, pp. 2G-7. 



(2 J Eye-pieces and Objectives. 



Hartnack's new Objective. — Wo transcribe tho following paragraph 

 verbatim :* — 



" A new objective, after calculations of Dr. Schroder, has been pro- 

 duced by Professor Hartnack, in Potsdam, whoso microscopic objectives 

 enjoy a well-deserved reputation, and which is destined to fill out the 

 place between the photographic aplanat and the microscopic system. The 

 weak microscopic systems, which are ordinarily applied, if more extended 

 microscopic objects, histological preparations, polished stones, and metals 

 are to be photographed, have besides their proportionate light-weakness 

 and their chemical focus, a very moderate expansion of the evenly 

 illuminated available picture field, comprising hardly more than 6 to 8 

 degrees. The small aplanats, which are used for the same purpose, 

 require very strong diaphragms and give a picture field with little plane. 

 The new objective, which is furnished witliout diaphragms, comprises an 

 extremely large picture angle of almost 26^, and covers to tlic edge of 

 the field with almost equal sharpness and without the least trace of 

 chemical focus. The instrument, which I have tested, has an equivalent 

 focal distance of about 50 mm., and forms a sharp object of nearly 4 sq. 

 cm. The light power is quite extraordinary ; for enlargements 10 to 15 

 times by ordinary 15-candle gaslight the exposure was 3 to 8 seconds 

 upon bromide of silver gelatin. The instruments, whose general intro- 

 duction is only to be desired, can also be executed in other sizes, as for 

 instance from 4 to 6 inches equivalent focal distance." 



Penny, W. G. — Eye-pieces— Physical Aberration and Distortion. 



Enjl. Mcch., XL VII. (1888) p. 215 (1 fig.). 



(3) Illuminating and other Apparatus. 



Hilgendorf's Auxanograph.t — This instrument, devised by Dr. 

 F. Hilgendorf, is a micropantograph designed to produce outline sketches 

 (orthogonal projections) of small objects down to less than 1 mm. on an 

 increased scale of from 2 to 10. 



The four arms W 6, W V, Z Y, X Y (fig. 103), are supported on long 



* Dr. 11. W. Vogel in ' Anthony's Photographic Bulletin,' 1888, p. 230. 

 t Zeitschr. f. Instrumentenk., vii. (1887) pp. 2'JO-l (1 fig.). 



