518 



MICROSCOPY. 



35. SPECTRO - POLAR- 

 IZER. 



glit, a collimating lens, a direct-vision prism, 

 and an ordinary objective to project a spectral 

 image of the slit at the plane of the object on 

 the stage. The spectrum is so distinct that 

 some hundreds of Fraunhofer lines can be seen, 

 and sufficiently intense with ordinary gas-light, 

 and a slit of O'Ol mm. wide, for the study of 

 bacteria under high powers. This apparatus is 

 easily extemporized by 

 placing a slit, collimat- 

 ing lens, and prism in 

 the sub-stage between 

 the mirror and a con- 

 denser-objective, like 

 *, 0, and /"beneath the 

 lens g in Fig. 35. The 

 spectro - polarizer of 

 Dr. A. Rollet, of Gratz, 

 is placed between the 

 illuminating mirror 

 and the object, for the 

 study of objects in both 

 monochromatic and 

 polarized light. It con- 

 sists of a Prazmowsld 

 polarizing prism Tc (Fig. 

 35), a slit at s adjust- 

 able by the milled head 

 d, a collimating lens 

 , a system of direct- 

 vision prisms f, whose 



spectrum is focused upon the object by the 

 lens g through the selenite plate c. The ana- 

 lyzing prism is placed above the ocular of the 

 microscope with an index moving over a cir- 

 cular scale attached to the ocular to show the 

 exact position of the prism. In using this in- 

 strument, the object is placed in one of the 

 dark interference bands caused by the selenite 

 plate, where, if it be doubly refracting even in 

 the slightest degree (like a particle of muscle, 

 for instance), it will appear dark in certain azi- 

 muths, but will become luminous in other azi- 

 muths in which its optical axis coincides with 

 that of the selenite plate (" A. M. M. J.," 1883,'p. 

 168). This extremely delicate and sensitive in- 

 strument can, like the foregoing, be readily ex- 

 temporized in connection with any good micro- 

 scope. For the same purpose, and to attain 

 still greater dispersion, Prof. Abbe, at the sug- 

 gestion of Dr. Dippel, has abandoned the direct- 

 vision prisms, and has devised an arrangement 

 for passing the light horizontally beneath the 

 stage through a polarizer, selenite plate, slit, 

 and achromatic collimating lens, to a pair of 

 dispersing prisms whose spectrum passes up- 

 ward through an objective at an angle of 90 

 from its original direction. The image of a 

 scale giving the wave-length of the lines in the 

 spectrum is projected by lenses in a supplement- 

 ary horizontal tube upon the upper surface of 

 the last prism, and by it reflected into the field 

 of view along with the spectrum. The polar- 

 izing prism is mounted in a movable frame 

 which can be turned aside when ordinary light 

 is required ("A. M. M. J.," 1883, p. 174). 



The " Society " screw, of the standard size 

 and proportions recommended in 1857 by the 

 London (now Royal) Microscopical Society as 

 a means of attaching objectives interchange- 

 ably to all microscopes, continues in practically 

 universal use in England and America, though 

 not usually attached to the smaller Continental 

 objectives except for purposes of export. The 

 extensive manufacture of the " Society " screw 

 by many manufacturers of various degrees of 

 skill, and possibly the employment of tools and 

 standards altered by wear, have led to such loss 

 of uniformity that the system no longer fully 

 accomplishes its object ; the objectives of some 

 reputable makers fitting too loosely into stands 

 of others, while the objectives of the latter en- 

 ter only partially or with difficulty into the 

 tubes of the former. To remedy this evidently 

 growing evil, the Royal Microscopical Society 

 is now issuing to manufacturers standard steel 

 gauges and taps designed to represent exactly the 

 official screw. These tools are now available and 

 are in general use ; but, although they presuma- 

 bly represent quite closely the theoretical "So- 

 ciety " screw, they prove better calculated to 

 guard against future errors than to harmonize 

 existing apparatus. In a paper on this subject 

 read by Mr. Edward Bausch at the American So- 

 ciety of Microscopists, in 1884 ("Proceedings," 

 p. 153), it was shown that three of these taps in 

 his possession were individually imperfect and 

 measurably unlike each other, and that many 

 objectives by various makers would not screw 

 easily if at all into an adapter cut by one of 

 them.* After the reading of this paper a com- 

 mittee was appointed to devise if possible a 

 plan for rendering such apparatus fully inter- 

 changeable. Meanwhile those who have instru- 

 ments cut with the standard taps must either 

 destroy their accuracy by having the screws 

 eased out by " rule of thumb," or else must 

 have no small portion of their objectives, am- 

 plifiers, etc., turned down to match, and thereby 

 rendered too loose for their original associa- 

 tions. 



The successful introduction of the "Society" 

 screw put a stop for many years to the use of 

 bayonet-catches, which were then being made 

 by Spencer, Grunow, and others for the ready 

 attachment of objectives, and to other labor- 

 saving devices for the same purpose. The 

 labor of manipulating the screw, however, led 

 to the employment of double, triple, and even 

 quadruple nose-pieces, by which any one of the 

 specified number of objectives could be rotated 

 promptly into the axis of the instrument. Be- 

 ing firmly made, and with angular arms to 

 hold the unused objectives out of the way of 

 the stage, they proved exceedingly useful in 

 cases where extreme precision was not re- 

 quired ; though analogous devices of inferior 

 model and workmanship had before been tried 



* Of 94 objectives, etc., by eighteen reputable makers, 

 tested by the writer, 23 by eleven makers were decidedly too 

 tipht, and 12 others by six makers would not screw up to the 

 shoulder at all. Those which failed were nearly equally di- 

 vided between English and American makers. 



