Prof. H. W. Dove on a New Photometer. 15 



pared are of different colours, or if a determination is to be made 

 of the brightness of light diffused in a given space, or if the 

 quantity of light is to be measured which a very small, or only 

 feebly transparent body transmits. In the latter case, the very 

 convenient method given by Bunsen for bright flames cannot be 

 applied : this method consists in illuminating by the lights a 

 grease-spot on a sheet of paper both in front and behind so that 

 it disappears. Babinet's method of neutralizing the colours of 

 polarization of two masses of light polarized at right angles to 

 each other also excludes the use of different-coloured lights, as 

 is directly evident from the bright colorific phenomena which are 

 produced in my dichrooscope*. The transformation of a posi- 

 tive Daguerreotype into a negative, when the reflected light pre- 

 ponderates over the diffused light which reaches the eye from 

 the same, which Pouillet has proposed, requires a room with 

 black walls. It has consequently only a limited application ; 

 moreover, its delicacy is not considerable if the objects to be 

 compared present small surfaces, inasmuch as all the masses of 

 light which fall diffused on the plate from all sources concur in 

 producing the phenomena on the Daguerreotype plate. The 

 juxtaposition of equally dark shadows of a rod, as devised 

 by Rumford, or of bright lines of light of a rotating luminous 

 knob, as in Wheatstone's method, excludes at once differently- 

 coloured sources of light, whose equal brilliancy the eye cannot 

 judge. The enfeeblement of light caused by crossed Nicols, glass 

 plates, or polarizing mirrors becomes an uncertain criterion in 

 feeble sources of light, if the measurement consists in judging 

 the actual disappearance, and not in the passage of one pheno- 

 menon into the opposite, which is essential in definite determi- 

 nations. The method I use has these advantages over those enu- 

 merated: — that it is very delicate; that it can be applied to objects 

 of any size in the same manner, whether they are brightly or 

 feebly luminous, of the same or different colours, and whether 

 transparent or opake; that, moreover, it is fitted for determining the 

 intensity of light of optical instruments, that it allows of several 

 different methods of measurement which mutually control each 

 other, and, lastly, that it is obtained by means of an instrument 

 which is in the hands of every working man of science. 



There are certain microscopic objects, for instance the skin of 

 an ephemera, which appear dark upon a bright ground when 

 illuminated from below, but bright upon a dark ground when 

 the illuminating mirror is covered. This is seen in a far more 

 beautiful manner in microscopic photographs, for instance in 

 Major Dickson's tablet in Rostherne Church, when it is looked 

 at in one of Schieck's microscopes with a power magnifying 

 * Phil. Mag. S. 4. vol. xx. p. 352. 



