convenience, represented 



5 present 

 I, gold, silver, arsenic, antimony, i 

 copper, potassium, sodium, barium, strontium, calcium, magnesium, aiu- 

 miuum, caesium and rubidium, at least those which lie between the lines 

 D and G. While the charts are very admirable and reliable, they still 

 leave much to be desired in a chemical point of view and for the purposes 

 of analysis, because a separate chart is needed for the spectrum of each 

 element to give a clear view of its optical characteristics. The author 

 does not state in what manner the elements were obtained in the state of 

 absolute chemical purity required by the peculiar nature of the problems 

 to be solved, nor have we the guarantee of Bunsen's name and author- 

 ity upon this most important point. Kirchhoff's apparatus consisted 

 essentially of four flint glass prisms, three of which had refracting angles 

 <^f 45°, the fourth an angle of 60°; the edges of the prisms could be 



scope completed the instrument. The distances of the lines were meas- 



was moved. The map of the spectrum is accompanied by a millimeter 

 scale with an arbitrary initial point; this renders the identification of the 

 lines ciisy. The map represents the spectrum as seen when the sun's alti- 

 tude is great, and the author has not inserted the lines due to atmospheric 

 absorption, though he frequently observed their great beauty especially in 

 the neighborhood of D. For the production of metallic spectra the au- 

 thor employed almost exclusively the electric spark, in consequence of the 

 great intensity of the light: a Ruhmkorff's coil was found advantageous, 

 in some cases" the lines have a measurable breadth and such cases are 

 noted bv a bracket in the drawing. The bright lines due to the passage 

 of the spark through the air were not very perceptible; the author has 

 represented of these only a group in the yellow and one in the green, but 

 the brightest of the metallic lines are represented. In some cases the 

 author observed coincidences of position in the lines of different metals : 

 he suggests that these may be only apparent and that separation might 

 be effected by a greater number of prisms, but it seems to us equally 

 probable that the metals examined may not have been absolutely pure. 

 The position of the maxima of light in the spectrum of any metal does 

 not depend on the temperature, the presence of other vapors, or upon any 

 condition other than the chemical nature of the vapor itself. On the other 

 hand the relative intensity of the different maxima depends not only on 

 the temperature but upon the changes in the mass of the vapor, so that 

 the appearance of the spectrum at different temperatures may be very 

 different. The inversion of the bright lines in metallic spectra produced 

 by passing an intense light through the flame containing the metallic 

 vapor is now a familiar fact. The author reters this fact to a general 

 physical principle, namely, that for every species of ray the ratio between 

 the power of radiation and the power of absorption is equal for all bodies 



