of Binocular Spectrum- Microscope. 385 



screw of the standard size, into which an object-glass of high power 

 is fitted. E represents one in position. I generally prefer a |-inch 

 power ; but it may sometimes be found advisable to use other powers 

 here. 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 objeet-glass E will form a small 

 image of the slit C, about - 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 II, 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 arrangement 

 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 or- 

 dinary 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 condenser 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 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 illuminated 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 aperture 0, 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 microscope-body at the place usually occupied by the 

 object-glass ; and the object-glass is attached by a screw in front of the 

 pnsm-box. It is shown in its place at L. The prism-box is suffi- 



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

 microscope could be made in its present complete form, I have been greatly as- 

 sisted 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 ar- 

 rangement of the different parts, so as to render them easily adapted to micro- 

 scopes of ordinary construe! ion. 



