PHYSICS. 4.83 



case the light leaving the hole and meeting the front surface of the lens 

 is refracted and meets the back surface normally. Most of the light 

 passes through ; but a small portion is reflected back along the path 

 whence it came, and is sufficient to produce an image easily visible in 

 the day. By fixing a plane surface of glass behind the lens it will be 

 found that anotlier image may be produced when the lens is about 

 twice as far from the card as it was before. The card is then at the 

 principal focus. {Phil. Mag., Jul}', V, xiv, p. 30.) 



Bedson and Williams have experimented to ascertain whether the 

 specific refrac"ion of a solid body can be obtained from that of its solu- 

 tions by applying the law of mixtures. Eocksalt, borax, boric acid, and 

 sodium-metaphosphate were the substances used. The indices cor- 

 responding to the three lines of hydrogen were measured for the three 

 bodies first named by cutting prisms from them ; for the fourth by im- 

 mersing it in a more refractive liquid, and adding a less refractive one 

 until equality was obtained. The results show a satisfactory accord- 

 ance. {J. Phys., August, 11, I, p. 377.) 



Crova has given the formula of a varnish for glass, given to him by 

 Guinand, so that drawings may be easily made upon it for purj)oses of 

 projection. It consists of ether, 500 grams; sandarac, 30 grams; mastic 

 in tears, 30 grams. When dissolved add benzene until the varnish 

 flowed on a plate of glass gives it a depolished appearance. To give 

 a fine grain and a homogeneous layer, pour upon it a little petroleum 

 ether, let it partially evajjorate, and rub with a piece of muslin until 

 perfectly dry. The coating may then be written upon either with ink 

 or a lead pencil, and is ready for use. {J. Fhys., January, II, i, p. 42.) 



Demeny has given an account of the application of instantaneous pho- 

 tograi)hy to the study of animal locomotion as practiced by Marey at 

 the physiological station of the Pare des Princes. The exposure is made 

 by a rotating opaque disk, having an opening in the form of a sector. 

 With a strong light it may be reduced to one thousandth of a second. 

 An image is obtained with each tuin of the disk, and since the object 

 has moved in the interval these will be neither identical nor superposed. 

 A wbite stone thrown by hand when thus photographed gives the im- 

 age of a parabolic curve. A white point attached to a carriage wheel 

 gives the cycloid described. {J. Phys., November, II, i, p. 504.) 



Stokes has studied the phenomena of the light border frequently 

 noticed in photographs just outside the outline of a dark body seen 

 against the sky. The sunlight, being reflected horizontally into a dark 

 room, is passed through a lens and allowed to fall on a phosphorescent 

 screen of calcium sulphide, i^reviously exposed to light, placed nearer 

 to the lens than its principal focus. There is seen a circular disk of 

 blue light much brighter than the general ground, separated by a dark 

 halo from this ground, due to the action of the less refrangible rays 

 which discharge the phosphorescence. But, since the first effect of 

 these rays is to produce light, there is seen, when the exposure is very 



