ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 101 



eye-cap under simultaneous use of the cube LB and the lens L, but 

 serves also to adjust the tube h Y with regard to the light to be measured. 

 When all the upper structure is in adjustment, LC produces on a 

 ground-glass disk n provided with a mark an approximately sharp, 

 image of the light-source to be measured. A mirror is set at v so that 

 the experimenter can conveniently observe the proper orientation of the 

 instrument. The screen c rotates on d by means of the external 

 handle g 2 . It is moved aside when the adjustment of LC is in process 

 but, on release, automatically resumes its first position and effectually 

 prevents the interference of any light from the observer's position with 

 that diffused through the revolver plates. The equation of observation 

 is B = c S, where B = the illumination strength in metre-candles, c = 

 the intensity, and S = the sector-opening as given on the graduated 

 scale at S. Then, if light of unit metre-candle is passed through //., 

 and if equality or equal contrast is obtained when S = 10, it follows 

 that c =0*1. If, the instrument remaining in the same adjustment, 

 illumination of 10 metre-candles is presented at ft, S would equal 100, 

 a number beyond the sector-range (graduated from 1 to 10). A plate 

 rotatory about C is now brought into the position m 2 , where it transmits 

 only • 1 of the light ; thus c now equals 1*0; in the position m 3 it 

 would transmit 0*01 of the light, and c would now equal 10. These 

 positions are all known by marks external to the chamber G-, and thus 

 by product of the values of r, and S the candle-power of an illuminant 

 is known. Further weakening of the light-source can be effected by 

 rotation of the tube r, which is fitted with windows of such a size that 

 they transmit 10 _1 , 10 ~ 2 , etc., of /x. For the measurement of smaller 

 illuminations a mirror of gypsum is placed obliquely before fx. The 

 diffuse reflecting power of gypsum is greater than the transmissibility of 

 the opal glass plates, and therefore the brightness of the source is 

 increased. Diminution of the comparison-light must be effected, if 

 necessary, by any of the means provided, and the calculation made as 

 before. 



Bell, L. — Physiological Basis of Illumination. 



[The author discusses many familiar difficulties of vision, e.g. the well 

 known trouble found at twilight in trying to work by a mixture of 

 natural and artificial lights.] 



Proc. Amer. Acad. Arts and Set., xlviii. (1907)pp. 77-96 (6 figs.) 



Eeprinted as a separate pamphlet. 



(4) Photomicrography. 



Turneretscher's Apparatus for Photomicrography.* — The full title 

 of G. M. Turneretscher's treatise is given below. The apparatus is the 

 outcome of many years' experience, and is adapted to the photography 

 of objects in their natural size, as well as to enlarged or diminished re- 

 productions. In all cases the apparatus lends itself to the easy deter- 

 mination of the proportion between object and image. The camera is 



* Apparate zur Herstellung von wissenchaftlichen photographischen Auf- 

 nahmen und von Mikrophotographien bei schwachen Vergrosserungen unter 

 bequemer Einhaltung eines genauen, Grossenverhaltnisses zwischen Objekt und 

 Bild. Museumskunde, iii. (Berlin, 1907) pp. 158-70 (4 figs.). Also as a separate 

 pamphlet. 



