ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 105 



rest to and fro. On the glass bottom of H is a minute photograph of 

 the graduated test type used by oculists. This is viewed through a small 

 lens F, adjustable at E, until a sharp image is seen by the observer. 

 When the cistern I is raised until the glass bottom of H and I touch, 

 the scale-reading on L then indicates zero. The depth of the liquid (as 

 indicated on the scale) required to produce complete extinction of the 

 light measures the intrinsic brightness of the source. Or with a con- 

 stant source of light the depth measures the " light threshold," or the 

 sensibility of the observer's eye to light. This sensibility rapidly rises 

 during the first minute of observation, and becomes nearly constant 

 after two or three minutes. The form sense, or " visual ocuity," of the 

 eye is measured by the depth of liquid required to obscure and produce 

 illegibility of the test type, and this also measures the illuminating 

 power of the source of light. The illuminating power of the source 

 may be reduced to any given fraction by means of the adjustable 

 and rapidly revolving sector, or by other means ; and it will lie found 

 that the depth of liquid required to produce extinctiou of the light 

 is practically the same, even when the illumination from the source 

 is reduced to a very minute amount ; in other words, the intrinsic 

 brightness remains the same. On the other hand, the legibility of 

 the test type varies with the amount of illumination, and it is this 

 we require to measure in the case of light irregularly reflected from 

 rough surfaces. Hence this arrangement affords an accurate method of 

 testing the illuminating power of any surface that scatters light, whether 

 large or small. It is only necessary to use a steady source of artificial 

 light, and note the depth of immersion of the plunger H which is 

 required to produce illegibility when a silvered mirror is employed ; 

 then replace or cover the mirror by a similar sized piece of the reflecting 

 surface to be tested, and note the depth now required for extinction, the 

 distance and intensity of the source of light remaining unchanged. 

 The author quotes the following as specimens of his results : — Silvered 

 glass, 100 ; plane glass surface, 65 ; ground glass, 45 ; white card, 45 ; 

 grey card, 35 ; dark grey card, 21 ; smooth black paper, 20 ; black 

 cotton cloth, 16 ; dull black woollen cloth, 5. 



An Adjustment for the Plane Grating similar to Rowland's 

 Method for the Concave Grating.* — C. Barus states that the remark- 

 able refinement which has been attained (notably by Ives and others) in 

 the construction of celluloid replicas of the plane grating, makes it de- 

 sirable to construct a simple apparatus whereby the spectrum may be 

 shown, and the measurement of wave-length made in a way that does 

 justice to the astonishing performance of the grating. He has therefore 

 devised an inexpensive contrivance in which the wave-length is strictly 

 proportional to the shift of the carriage at the eye-piece : which for the 

 case of a good 2 -metre scale divided into centimetres admits of a mea- 

 surement of wave-length to a few Angstrom units, and with a millimetre 

 scale should go much further. Observations are throughout made on 

 both sides of the incident rays, and from the mean result most of the 

 usual errors should be eliminated by symmetry. 



* Proc. Amer. Phil. Soc, xlviii. (1909) pp. 166-76 (5 figs.). 



