228 Transactions of the Society. 



Near the top of this, a small piece, £ in. square, of a Thorp diffrac- 

 tion grating film of 14,000 lines per inch, is fixed between two 

 pieces of glass covered with a black cardboard diaphragm having a 

 hole of j? -in. diameter. The distance between the grating at the 

 upper end and the slit at the lower end is 8 to 10 in., or, if desired, 

 the tubes may be made to slide so that the distance can be varied. 

 This constitutes the whole instrument ; it will be observed that 

 no lenses are employed at all. 



To use the spectroscope to compare strips of coloured film, for 

 example, the latter are kept close to the slit with a couple of 

 elastic bands, and on looking through the grating at the eye end 

 of the instrument, the actual colours of the films are seen behind 

 the slit, flanked on each side by spectra of the colours in question. 

 As it is only needful to view one of these sets of spectra (the 

 other being superfluous), it is convenient to keep the slit near the 

 right side of the object end of the tube. The left-hand set of 

 spectra can then be view T ed comfortably without any eye-strain, 

 whilst the right-hand spectra move outside the field of vision. 

 Xearly all gratings yield a better and brighter spectrum on one 

 side than the other; the grating should therefore be turned round 

 so that its best spectrum appears on the left-hand side. 



To view dye solutions in test-tubes, it is of course only necessary 

 to hold the spectroscope so that the slit lies horizontally ; the 

 spectra are then viewed below the slit. 



The two special advantages of the instrument, apart from its 

 simplicity and inexpensivenes?, are: — 



1. That as many as half a dozen different spectra can be viewed 

 and compared at the same time, together with white light. 



2. That the colours of the objects themselves are seen at the 

 same time as well as their spectra. 



I do not know of any other form of spectroscope which possesses 

 these conveniences, and can testify to the utility and efficiency 

 of the little apparatus in question from frequent use during a 

 number of years. 



Eecently, in the course of some research work, it has been 

 necessary for me to examine and compare the spectra of the 

 colours of the microscopic colour-lines and dots of multicolour 

 screen-plates used for colour-photography, and for this purpose of 

 course the apparatus described above is unsuited. It therefore 

 seemed desirable to devise some form of micro-spectroscope 

 possessing the same advantages, and the following extremely 

 simple form, which anyone can fix up for himself in a few 

 moments, is the outcome. 



An ordinary low-power eye-piece is taken, and in the plane of 

 the diaphragm a stop is inserted with a slit about 1 mm. width, 

 taking care that the slit is near the right-hand side of the circular 

 hole of the diaphragm. A small piece of Thorpe grating film, 



