ZOOLOGY AND BOTANY, MICROSCOPY, ETC. r)27 



For the purpose of tlius obtainiiiii^ a readiiif,^ which sii])divides tlie 

 scale-division, a dark field is provided immediately al)ove the scale. 

 This dark field is merely a very narrow black band stretched across the 

 field of the instrument. It does not in any way interfere with the use 

 of the micrometer wire in taking the reading. 'But when the diffrac- 

 tion grating is brought into position over the eye-lens this black 

 band fades to a light grey, and on it are seen images of the scale and of 

 the micrometer wire, which appear to be prolonged across its breadth. 

 By rotating the diffraction grating through a small angle about the 

 optical axis these images are made to travel lengthways of the band, and 

 forasmuch as the image of the micrometer wire traverses the band in one 

 direction and the image of the scale in the opposite direction, a position 

 is soon reached in which the image of the micrometer wire coincides 

 with the image of the adjacent scale ruling. The position of the 

 diffraction grating which yields this result affords a measure of the out- 

 standing fraction of a scale-division which expresses the position of the 

 micrometer wire. In this way the scale is subdivided. 



Two methods are available for reading this fraction. Thus, it may be 

 read on a subsidiary scale placed below the principal scale in the field of 

 the instrument, or, if the operator objects to the presence of this sub- 

 sidiary scale, then the fraction can be read upon a segment divided upon 

 the flange of the cell containing the eye-lens. 



The" cap in which the diffraction grating is mounted is made easily 

 removab le, so that in the ordinary use of the ocular there are no diffrac- 

 tion images to disturb the observer's attention. The diffraction grating 

 is only brought into use when an accurate reading of the position of the 

 micrometer wire is desired. It may be remarked, however, that these 

 diffraction gratings are so designed as to give extremely feeble images 

 only on the bright field of the instrument, images so feeble that they do 

 not seriously affect the making of an observation. The objection to 

 their presence, though very intelligible, is entirely a question of "nerves." 

 For this reason it is desirable that the diffraction grating should be 

 altogether removable for ordinary use. When employed it yields images 

 which flash out dead black at the moment of super-position, and thus 

 make the taking of an exact reading in this way not only easy but 

 certain. 



(3) Illuminating: and other Apparatus. 



The Insectoscope.* — The above name is applied by P. Marie to an 

 apparatus which he has contrived for facilitating the examination in 

 reflected light of an object in relief. His purpose is to obviate the 

 well-known difficulties met with in the examination by a simple lens of 

 an insect or other such object. While with a loop it is scarcely possible 

 to apply a magnification greater than 20 diameters, his apparatus adapts 

 itself to 80. When the object has been once fixed on its support in the 

 Microscope this support can be moved, without loss of optic centre, in 



* Bull. Soc. d'Encouragement pour I'lndustrie Nationale, cxix. (Paris, 1913) 

 pp. 638-45 (3 figs.). 



2 T 2 



