966 SUMMABY OF CUERENT RESEARCHES RELATING TO 



proceed witliout deviation, but the extraordinary ray, whose index is 

 between 1-483 and 1*658, according to the direction of the ray, will 

 not enter the spar if the angle of incidence is suitably regulated. 

 My prism is devised to fulfil these conditions. 



A polarizing prism is thus obtained about equal in length to that 

 of Hartnack and Prazmowski, but the exterior field of view is 

 44° 46' 20". A large piece of spar is not required, a simple cleavage 

 plate suffices ; moreovei', as the end-faces are of flint glass, they may 

 be cleaned without injury. 



This form of construction may be still further improved: the 

 flint-glass prism maybe cut in a plane forming an angle of 63° 26' 15" 

 with the end-faces, and a cleavage plate of spar inserted between the 

 section-faces as before. This prism is again cut in a plane sym- 

 metrical with the former in relation to the axis, and the two parts 

 are cemented together, having between them another cleavage plate 

 of spar placed symmetrically in relation to that in the first section. 

 We thus obtain a polarizing prism half the length of Nicol's, with a 

 field of view of 98° 41' 30"." 



Electric Illumination for Anatomical, Microscopical, and Spec- 

 troscopical Work.* — Dr. C. von Voit describes the result of some 

 experiments as to the electric light, conducted by himself and Drs. 

 Kiihne, Kupflfer, Eiidinger, and Bollinger. 



The lamps used were an Edison incandescent lamp, of about 16 

 candle-power, a Miiller, of about 24, and a Maxim of from 36 to 60, 

 respectively. 



In every instance the light was sufficient for the finest micro- 

 scopical observations, and for the highest magnifying powers, free 

 from the well-known disadvantages of other artificial illumination, 

 such as the preponderance of the yellow-rays, and the heat with close 

 approximation. "When the light of the Maxim lamp was raised to 

 about 60 candles, so that the M-form of the carbon filament was 

 unrecognizable in consequence of the irradiation, the heat was 

 scarcely perceptible, when the face was within 25 cm. of it. 



The 16-candle lamp was effective at a distance of 1 m. For the 

 arrangement of many Microscopes in a circle at a convenient distance 

 round the source of light, the Miiller lamp is most to be recommended, 

 because the spiral form of the carbon produces equal effects in all 

 directions. 



The greatest intensity — of 60 candles — was equivalent to the best 

 available diffused daylight, when the rays were made parallel by a 

 condenser before falling upon the mirror. 



In all the observations it was necessary for obtaining homogeneous 

 images, unaffected by any reflex and interference phenomena, to insert 

 immediately under the object a plate of ground-glass, or to place the 

 preparation upon the polished side of a ground-glass slide. 



* Oentral-Ztg. f. Optik u. Mechanik, iv. (1883) p. 206. Aus Die Elektro- 

 Medicin in der Internat. Elektr.-Ausst. zu Miinchen im Jahre 1882 von Dr. E. 

 Stintzingr. 



