of Binocular Spectrum- Microscope. 387 



box, and the large aperture D being brought into the centre of the 

 substage. When it is desired to examine the spectrum of any por- 

 tion of an object in the field of view, all that is necessary is to push 

 the slit into adjustment with one hand, and the prisms with the 

 other. The spectrum of any object which is superposed on the image 

 of the slit is then seen. 



The small square aperture at O (fig. 1) is for the examination of 

 dichroic substances. When this is pushed into the field, by placing 

 a double-image prism P between A B and E, two images of the aper- 

 ture are seen in juxtaposition, oppositely polarized ; and if a dichroic 

 substance is on the stage, the differences of colour are easily seen. 



When the spectrum of any substance is in the field and the double- 

 image prism P is introduced, two spectra are seen, one above the other, 

 oppositely polarized, and the variations in the absorption-lines, such 

 as are shown by didymium, jargonium, &c, are at once seen. 



A Nicol's prism, Q, as polarizer, is also arranged to slip into the 

 same position as the double-image prism, and another, R, as analyzer, 

 above the prism-box. The spectra of the brilliant colours exhibited 

 by certain crystalline bodies, when seen by polarized light, can then 

 be examined. Many curious effects are then produced, a description 

 of which I propose to make the subject of another paper. Both the 

 prisms P and Q are capable of rotation. 



If the substance under examination is dark coloured, or the illumi- 

 nation is not brilliant, it is best not to divide the light by means of 

 the Wenham prism at N, but to let the whole of it pass up the tube 

 to one eye. If, however, the light is good, a very great advantage 

 is gained by throwing the Wenham prism into adjustment and using 

 both eyes. The appearance of the spectrum, and the power of grasp- 

 ing faint lines, are incomparably superior when both eyes are used ; 

 whilst the stereoscopic effect it confers on some absorption and in- 

 terference spectra (especially those of opals) seems to throw entirely 

 new light on the phenomena. No one who has worked with a ste- 

 reoscopic spectrum- apparatus would willingly return to the old mo- 

 nocular spectroscope*. 



If the illumination in this instrument is taken from a white cloud 

 or the sky, Fraunhofer's lines are beautifully visible ; and when using 

 direct sunlight they are seen with a perfection which leaves little to 

 be desired. The dispersion is sufficient to cause the spectrum to fill 

 the whole field of the microscope, instead of, as in the ordinary in- 

 strument, forming a small portion of it, the dispersion being four or 

 five times as great ; whilst, owing to the very perfect achromatism 

 of the optical part of the microscope, all the lines from B to G are 

 practically in the same focus. 



As the only portion of the object examined is that part on which 

 the image of the slit falls, and as this is very minute (varying from 



* It is not difficult to convert an ordinary spectroscope into a binocular instru- 

 ment. The rays after leaving the object-glass of the telescope are divided into 

 two separate bundles and received on two eyepieces properly mounted. As it is 

 immaterial whether the spectrum be stereoscopic or pseudoscopic, a simpler form 

 of prism than Mr. Wenham's arrangement can be used. 



