460 E. EAT LANKBSTEE. 



ment by which the light from a powerful incandescent lamp is 

 passed through two parallel slits, A and B, giving in the field 

 of view of the eye-piece spectroscope two spectra exactly 

 parallel to one another, and of exactly equal intensity of light. 

 A diaphragm is made to traverse the field by the turning of a 

 screw, 80 as to present for observation a narrow band only of 

 the juxtaposed spectra. The exact wave length of this strip 

 is given by a scale introduced. Thus a width of the spectrum 

 corresponding to a range of only some two or three millionths 

 of a millimetre in wave length and of measured position in 

 the spectrum can be examined, whilst there is on either side 

 absolute darkness. The portion of the strip of light thus 

 studied belonging to the light coming through slit A can be 

 compared, as to the amount of light present, with the identical 

 representative portion of the spectrum belonging to slit B. 

 Under the conditions so far stated, the intensity of illumina- 

 tion (amount of light) of each half of the strip (that belonging 

 to spectrum of slit A, and that belonging to spectrum of slit 

 B) are exactly and sensibly equal. If now there be placed in 

 front of slit A a coloured transparent body, some of the light 

 passing through that slit will be stopped. Suppose the travel- 

 ling eye-piece diaphragm is adjusted so as to present to the 

 observer a strip of each spectrum corresponding to a wave 

 length which is partially absorbed by the coloured medium 

 introduced before slit A, then the portion of the strip 

 belonging to slit A will be sensibly dimmer than that belong- 

 ing to slit B. 



Now by a micrometer screw Engelmann can reduce the 

 width of slit B until the amount of light coming through that 

 slit is no greater than the amount coming through slit A, ob- 

 scured as it is by the coloured medium. The amount of 

 movement of the screw required to bring the light of slit B 

 down to the intensity of that of slit A furnishes the measure- 

 ment of the absorption due to the coloured medium for the 

 wave length under observation (isolated by the travelling eye- 

 piece diaphragm slit). 



The micrometer screw is standardised so as to give readings in 



