540 
ME. W. HUGGINS ON THE SPECTEA 
together with copper bands. These plates are fixed into varnished plates of wood, which 
form covers to the cells. In each wooden cover is a slit of the length of the width of the 
carbon plates, by which an amalgamated zinc plate can be inserted between the plates of 
graphite. An important part of the arrangement is a third large cell of ebonite, which 
is filled with water acidulated with a few drops of sulphuric acid, and contains at the 
bottom some mercury. As soon as a plate is removed from the battery after use, it is 
rubbed clean under running water, and then immersed in the spare cell until it is again 
required. By this arrangement the plates are always clean and perfectly amalgamated. 
The solution employed to charge the battery is a saturated solution of bichromate of 
potash, to which about yg- part of sulphuric acid has been added. The battery is suffi- 
ciently powerful and always ready for instant use. For the convenience of varying the 
power of the battery, three sets of zinc plates are kept in the spare cell ; the plates are 
2 inches, 4 inches, and 6 inches in width. 
§ III. Observations of Nebulas. 
For the greater convenience of reference and of comparison, the spectrum of 37 H. IV. 
Draconis from my paper “ On the Spectra of some of the Nebulae”*, has been added to 
fig. 2, Plate XXXIII. The spectrum of this nebula may be taken as characteristic, in its 
general features, of the spectra of all the nebulae which do not give a continuous spectrum. 
At present I have determined satisfactorily the general characters of the spectra of about 
seventy nebulae. This number forms but a part of the much larger list of nebulae which 
I have examined, but in the case of many of these objects their light was found to be 
too feeble for a satisfactory analysis. Of these seventy nebulae about one-third give a 
spectrum of bright lines. The proportion, which is indicated by this examination, of 
the nebulae which give a spectrum of bright lines to those of which the spectrum is 
continuous (namely, as one to two), is probably higher than would result from a wider 
observation of the objects contained in such catalogues as those of Sir John Herschel 
and Dr. D’ Arrest, since many of the objects which I examined were specially selected, 
on account of the probability (which was suggested by their form or colour) that they 
were gaseous in constitution. 
All the differences which I have hitherto observed between the spectra of the gaseous 
nebulae may be regarded as modifications only of the typical form of spectrum which is 
represented in the diagram, since they consist of differences of relative intensity, of the 
deficiency of one or two lines, or of the presence of one or two additional lines. It is 
worthy of remark that, so far as the nebulae have been examined, the brightest of the 
three lines, which agrees in position in the spectrum with the brightest of the lines of 
the spectrum of nitrogen, is present in all the nebulae which give a spectrum indicative 
of gaseity. It is a suggestive fact that should not be overlooked, that in no nebula 
which has a spectrum of bright lines, has any additional line been observed on the less 
refrangible and brighter side of the line common to all the gaseous nebulae. 
* Philosophical Transactions, 1864, p. 438. 
