LOCKYEK.— USE OF THE SPECTROSCOPE. 29 



Instructions for use of Instrument A. 



The stand, with its train of prisms, should be taken out of the 

 box and placed on a firm support. The prisms if dusty should 

 then be Aviped with a soft brush or leather, care being taken, if the 

 leather be used, to move it in one direction as little as possible, 

 to prevent scratches. The tube furnished with a focussing screw 

 should, after the adjustment referred to in the next paragraph, be 

 screwed into the ring which moves along the arc. The other tube 

 should be fixed into the other fixed ring. 



To focus the observing telescope insert the eye-piece to be 

 used, and obtain an image of a distant object, using a piece of 

 glass, of the colour of that part of the spectrum to be observed, 

 between the eye-piece and the eye. When this has been done and 

 both telescope and collimator fixed in position, the image of the 

 slit should be brought into focus by using the sliding tube of 

 the collimator. 



The slit plate and slipping piece should be so inserted into the 

 collimator tube that the adjusting screw of the slit will be on the 

 side nearest the observer. 



The whole train of prisms should be covered with black velvet. 

 This may be supported by a piece of thick cardboard (blackened) 

 resting on the two telescopes, and train of prisms in order to 

 prevent it from intercepting any of the light. 



Coloured glasses are supplied in order to prevent the observa- 

 tions of each part of the spectrum being interfered with by the 

 presence of stray light of any other colour than that under obser- 

 vation ; these should be placed in front of the slit. 



It will be well to commence all series of observations of the 

 region B — A at the more refrangible side of B, noting micrometer 

 reading, both number of revolutions and j-l^ths, in each case. B 

 will then serve as a zero. It will be found that the slit will bear 

 opening without much loss of definition as the less refrangible 

 region between a and A is approached. 



The less refrangible line of D and 6, and F and C themselves, 

 may be used in the same manner for observations in the regions 

 of the spectrum near these lines. 



To render the light more intense lenses are supplied to throw an 

 image of the sun or an intense parallel beam through the slit : 

 the sun can be followed by the tangent screw movement fixed to 

 the support. The height of the sun should be recorded against 

 each observation, and this can be facilitated by keeping the colli- 

 mator carefully levelled, marking degrees on the slit plate and 

 observing where the image of the sun falls. 



It will be observed that in Angstrom's maps the gradations of 

 the efi'ect of absorption from maximum (above) to minimum 

 (below) are shown. In Mr. Hennessey's maps these gradations are 

 absent. It will be well to prepare a wide spectrum, on a scale 

 about equal to that employed by Mr. Hennessey, showing the 

 gradations and the variations, if any, from those observed by 

 Angstrom. 



