452 On a New Form of Direct Vision Specti*oscope. [Dec. 9, 



For greater convenience in manipulation, and at the same time to 

 ensure a large angular aperture, our instrument has a short collimator 

 and telescope. The magnification of the image is in consequence but 

 small, but nevertheless it is easy to see the nickel line between the 

 two D lines in the solar spectrum, and the large angular aperture 

 makes it easier to see faint spectra from large objects, as well as to 

 catch the light of a moving object. With an eyepiece of low power it 

 serves well for the observation of absorption spectra. 



Measurements are made with it by bringing the line of which the 

 position is to be measured to a fixed pointer in the middle of the field 

 of view. The whole angle through which the prisms have to be 

 moved in passing from A to H is 10° 15', and as it is easy to read 

 quarter minutes with the vernier, considerable accuracy of measure- 

 ment may be attained. It must be observed, however, that the 

 change of angle is not proportional to the change of refrangibility of 

 the rays brought to mid-field, because a larger proportional rotation 

 is required at the violet end, when the prisms are near the position 

 of minimum deviation, than at the red end. Still there is a rotation 

 of 33' 30" in passing from A to B. 



The prisms are enclosed in a box, and though this is an advantage 

 in viewing faint spectra, it would sometimes be difficult to see the 

 pointer without some means of illuminating either the pointer or the 

 field. We effect the latter object by a slit in the side of the box, and 

 a small white paper reflector, ah in the figure, which throws light 

 from the opening in the box on to the side of the object glass, 

 clear of the prism. The slit in the box may be closed with a shutter 

 or with glasses of different colours. 



Lastly, the instrument can be used either with or without a stand. 

 Without the stand it is light enough to be held in the hand and 

 directed to the sky or to a moving object. 



It has been constructed for us by Mr. Hilger with his usual skill. 



