482 On the Spectral Phenomena of Opals. [Oct:, 



the axis of the microscope, -whilst screws G and h, at right angles to 

 each other, bring the image of the slit into any desired part of the 

 field. If the brass slide a b is pushed in so as to biing the circular 

 aperture d in the centre, the substage arrangement then becomes 

 similar to the old form of achromatic condenser. Beneath the sht 

 is an arrangement for holding an object, ia case its surface is too 

 irregular, or substance too dense, to enable its spectrum to be 

 properly viewed in the ordinary way. 



Suppose an object is on the upper stage of the microscope, 

 and viewed by Hght transmitted from the mirror through the large 

 aperture d and the condenser e; by pushing in the lower brass 

 slide A B so as to bring the slit c in the field, and then focusing by 

 the milled head f, it is evident that a luminous image of the slit c 

 can be projected on to the object ; and by proper adjustment of the 

 focus, the object and the slit can be seen together equally sharp. 

 Also, since the whole of the light which ilhmiinated the object has 

 been cut ofi", except that portion which passes through the slit, all 

 that is now visible in the instrument is a narrow luminous line, in 

 which is to be seen just so much of the object as falls withia the 

 space this line covers. By altering the slit adjustments the length 

 or width of the luminous line can be varied, whilst by means of the 

 rackwork attached to the upper stage any j)art of the object may 

 be superposed on the luminous line. The stage is supphed with a 

 concentric movement, which permits the object to be rotated whilst 

 in the field of view, and thus allow the image of the slit to fall on 

 it in any direction. During this examination a touch with the 

 finger will at any time bring the square aperture o, or the circular 

 aperture d, into the field instead of the slit, so as to enable the 

 observer to see the whole of the object ; and in the same manner 

 the slit can as easily be again brought back into the field. 



The other essential part of this spectrum microscope consists of 

 the prisms. They arc of the direct vision kind, consisting of three 

 flint and two crown, and are altogether 1 • 6 inch long. The box k, 

 holding them, screws into the end of the microscope body at the 

 place usually occupied by the object-glass, and the object-glass l is 

 attached by a screw in front of the prism-box. The prism-box is 

 sufficiently wide to enable the prisms being pushed to the side when 

 not wanted, so as to enable the light, after passing through the 

 object-glass, to pass freely up the body of the instrument. A pin 

 M enables the prisms to be thrown either in or out of action by a 

 movement of the finger. As the prisms are close above the object- 

 glass, the sliding-box n, carrying the Wenhani binocular prism and 

 the Nicol's prism, may be employed as usual, and the spectrum of any 

 substance may thus be examined by both eyes simultaneously, either 

 by ordinary light or when it is under the influence of polarized 

 light. The insertion of the prism-box between the object-glass and 



