June 17, 1897] 



NATURE 



157 



language by comparing their positions with those we 

 observe in the spectra of terrestrial light sources. 



But a straight slit is not the only kind of aperture we 

 can employ ; we may replace it by a ring, for instance. 



What we shall see in passing from the spectrum of a 

 candle to the spectrum of a spirit-lamp flame with salt m it, 



pensed with. A star as a point is part of a line slit ; 

 hence the success of Fraunhofers arrangement for 

 observing stellar spectra, to which I have referred. In 

 an eclipse we get a bright narrow ring round the dark 

 moon. There is our ring slit. Hence the so-called 

 " slitless spectroscope," or " prismatic camera," as it is 



Fig. 4. — The spectra of continuous and discon- 

 tinuous light sources, the latter seen with a 

 line and circular slit. 



Fig. 5. — The spectr 



of a complicated light-source as seen with a circular and 

 line slit. 



using first a straight and then a circular slit, is shown in 

 the accompanying woodcut (Fig. 4). 



If we examine a very complicated light source we shall 

 arrive at the same result, a spectrum characterised by a 

 large number of bright circles or lines (Fig. 5). 



called when photography is employed, used to study the 

 spectra of stars and the sun's chromosphere during 

 eclipses. 



The way in which, in the prismatic camera, the prism is 

 fixed outside the object-glass, is shown in the accompany- 

 ing figure (Fig. 6). 



We are now in a position to inquire how this 

 arrangement has been used during eclipses since 

 1871. J. Norman Lockyer. 



• {To be continued.) 



Fig. 6. — Details of objective prism. 



We have seen that in an ordinary spectroscope, when 

 we are studying light sources close to us, the rays have 

 to be made parallel before they pass through the prism. 

 But the heavenly bodies are at such a distance from us 

 that their light reaches us in a parallel beam, so that one 

 part of the spectroscope, the collimator, may be dis- 



NO. 1442, VOL. 56] 



THE 



OF 



OLD RED SANDSTONE 

 LORNE. 

 T^HE terraced hills of Lome, though a familiar 

 -*■ feature in the scenery of western Argyll- 

 shire, have not yet had their geology properly 

 worked out. Their peculiar topography, however, 

 has long been known to arise from the outcrops of 

 successive sheets of lava, lying upon and interca- 

 lated with strata of purple shale and conglomerate. 

 The age of these strata has never been satis- 

 factorily settled. For many years past I have 

 regarded them as probably belonging to the Lower 

 Old Red Sandstone, for their lithological characters 

 present a close resemblance to those of the great 

 basin of that age in central Scotland, which I 

 have distinguished by the name of " Lake Cale- 

 donia." While recently mapping in the district, 

 Mr. R. G. Symes, of the Geological Survey, came 

 upon an exposure of the sedimentary rocks, which 

 seemed to him so promising a locality for fossils 

 that he requested the assistance of one of the 

 fossil-collectors of the staff with a view to more 

 minute examination. Accordingly, Mr. A. Mac- 

 conochie was detailed for the purpose, and was 

 soon rewarded by the discovery of undoubted 

 remains of plants and fishes. The specimens 

 were first sent to the Office of the Survey in 

 Edinburgh, where the plants were recognised by 

 Mr. B. N. Peach as portions oi Psilop/iyfon, and 

 the fish-remains as parts of Cephalaspis. His 

 identification of the ichthyolites was immediately 

 confirmed by Dr. R. H. Traquair. The specimens 



were then forwarded to the Jermyn Street Museum, 



where they have been again examined by Mr. E. T. 



Newton, who entirely agrees with the opinion already 



pronounced regarding them. 



The occurrence of a genus of plants and another of 



fishes so characteristic of the Lower Old Red Sandstone 



