428 C. PIAZZI SMYTH ON MICEOMETRICAL MEASURES OF 



magnify ings up to a size fully 12 times larger than that of the Swedish 

 Philosophers. These lines moreover in the vacuum tubes, are very distinctly 

 double, and get wider and wider in their duplicities, though they decrease 

 their whole distances of double from double, with every succeeding line. 



There must therefore, in their omission, be an error in the work of those 

 otherwise unexceptionable authorities; and it is pretty certainly owing in large 

 part to the bad definition, or broad slit of the spectroscope there employed, 

 as well as the greater practical difficulties in the positive mode of representing 

 bright-line spectra. For the Upsala linelets are ultra hazy things, running one 

 into the other and making only a confused and slightly undulating surface of 

 luminous fog ; culminating too soon, on the red-ward side, into the perfect 

 light, or wdiiteness, of white paper. Whereas in my Vacuum tubes, far beyond 

 the Blow-pipe's hazy separations already alluded to, the linelets, however faint, 

 are thin and linear ; and in some new tubes are capable of exquisite sharpness 

 of definition, on an almost absolutely black field of view. 



' That however is not all that has to be noted with a very high Dispersion 

 power ; for after further working these tubes, the linelets became double, and 

 after that even treble ! Such a change however being always the beginning of 

 a tube failing, or going altogether wrong: and was first testified to, as will be seen 

 in Plate No. LIIL, by the very superior optical power of a fine Grating which 

 I had the honour of receiving from Professor Rowland, of Johns Hopkins 

 University, Baltimore, U.S. ; but corroborated afterwards only too abundantly 

 by the older prismatic apparatus. Hence some advantage will be found, when 

 comparing my different views of any of these CH bands, to note the name of 

 the tube employed on the occasion, and the date of observation ; a single clay, 

 of hard work, often showing great progress in the work of deterioration. 



Orange Band of CH ; and Effects of Pressure, in Vacuum 



Tubes so-called. 



For testing my own views of any other than the Citron and Green bands 

 just disposed of, — we must fall back on the general map of the Upsala 

 scientists, small though it be. But it is beautifully engraved ; in the negative 

 manner fortunately as to representing light by black ; and professes to give 

 the Orange, Citron, Green, Blue and Violet bands. Shading them, however, 

 into striking relief by adding to others closely ruled vertical lines, which are a 

 mere engraver's easy method of producing shade, and mean nothing, while they 

 mislead much, in spectroscopy. 



Comparing it, however, first of all with my own Index Map of CH in 

 vacuum tubes (Plate LXXVII.) — what is the meaning of the immense force of 

 the Orange band, and at the same time the dwindling down to a mere trace of 



