74 PROFESSOR J. N. LOCKYER ON THE PHOTOGRAPHIC 
clock, which was not then completely finished. One plate was exposed for four hours, 
on February 11, 1890, but, unfortunately, in consequence of the high wind, the slit 
was covered for an unknown part of this time by the velvet used to keep out stray 
light, and this was not at once discovered, as the finder for directing the telescope is 
at the lower end of the reflector tube, away from the spectroscope. This photograph 
only shows three or four of the more prominent lines, but they are all sharply defined. 
The other photographs were taken on February 2, 8, 9, and 10, the last with an 
exposure of three hours. 
As a collimator has not yet been fitted to the tube of the reflector the exposure of 
the plate to the flame of burning magnesium was made by closing the mirror cover, 
and burning magnesium at its exact centre. One half of the slit was exposed to the 
nebula, and the other half to the burning magnesium. 
The part of the nebula photographed was the bright portion in the region of the 
trapezium. In some photographs, in consequence of clock irregularities, the stars 
of the trapezium have imprinted their spectra upon the plates, but these in no way 
interfere with the spectrum of the nebula, since a longish slit was used, and the 
spectra of the stars are narrow. 
There is a remarkable and almost absolute similarity between the photographs 
obtained. The best one, taken on February 10, shows all the lines of the other 
photographs with cthers in addition, and this has therefore been selected for the 
determination of wave-lengths. 
The probable mean position of the slit during the three hours’ exposure of this 
photograph is shown in fig. 1, but the irregularities in the driving caused all the 
stars in the trapezium to cross the slit at different times. 

Fig. 1. Showing mean position of slit in photograph of February 10, 1890. 
It has not been found possible to reproduce the negative with advantage in 
consequence of its small size, but fig. 3 (see p. 80) gives a good idea of the appearance 
of the eleven principal lines shown in the photograph, and the position of the stellar 
spectra on the plate. Further reference to this diagram will be made later. 
The principal lines are the three ordinarily seen in the visible spectrum, the lines of 
