1869.] 



Binocular Spectrum-Microscope. 



447 



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 aperture are seen in juxta- 

 position, 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 crys- 

 talline 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 illumination 

 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 grasping faint lines, are incomparably 

 superior when both eyes are used ; whilst the stereoscopic effect it confers 

 on some absorption and interference spectra (especially those of opals) 

 seem to throw entirely new light on the phenomena. No one who has 

 worked with a stereoscopic spectrum- apparatus would willingly return to 

 the old monocular 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 sun- 

 light 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 instrument, forming a small por- 

 tion 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 0*01 to 



* It is not difficult to convert an ordinary spectroscope into a binocular instrument. 

 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. Wen- 

 ham's arrangement can be used. 



