1869.] 



Binocular Spectrum-Microscope. 



445 



The slit C and the object glass E are about 2 inches apart ; and if light is 

 reflected by means of the mirror along the axis of the instrument, it is 

 evident that the object-glass E will form a small image of the slit C, about 

 0*3 inch in front of it. The milled head F moves the whole substage up 

 or down the axis of the microscope, whilst the screws G and H, at right 

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

 of the field. If the slide A B is pushed in so as to bring the circular 

 aperture D in the centre, the substage arrangement then becomes similar 

 to the old form of achromatic condenser. Beneath the slit C is an arrange- 

 ment for holding an object, in case its surface is too irregular, or substance 

 too dense, to enable its spectrum to be properly viewed in the ordinary 

 way*. 



Supposing an object is on the upper stage of the microscope (shown in 

 fig. 2) and viewed by light transmitted from the mirror through the large 

 aperture D and the condeuser E, by pushing in the slide A B so as to 

 bring the slit C into the field, and then turning the milled head F, it is evi- 

 dent 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 illumi- 

 nated the object has been cut off, 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 within 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 part of the object may be superposed on 

 the luminous line. The stage is supplied with a concentric movement, 

 which permits the object to be rotated whilst in the field of view, so as to 

 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 aper- 

 ture 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 into the field. 



The other essential part of this spectrum-microscope consists of the 

 prisms. These are enclosed in a box, shown at K (fig. 2). The prisms 

 are of the direct-vision kind, consisting of three flint and two crown, and 

 are altogether 1*6 inch long. The box screws into the end of the micro- 

 scope-body at the place usually occupied by the object-glass ; and the ob- 

 ject-glass is attached by a screw in front of the prism-box. It is shown in its 

 place at L. The prism -box is sufficiently wide to admit of the prisms being 

 pushed to the side when not wanted, so as to allow the light, after passing 



* In carrying out the experiments -which were necessary before this spectrum-micro- 

 scope could be made in its present complete form, I have been greatly assisted by Mr. C. 

 Collins, Philosophical-Instrument Maker, 77 Great Tichfield Street, to whom I am also 

 indebted for useful suggestions as to the most convenient arrangement of the different 

 parts, so as to render them easily adapted to microscopes of ordinary construction. 



