684 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Herr R. Winkcl * has endeavoured to remove this objection by 

 leaving the eye-lens in a fixed position and moving the micrometer 

 vertically hy the contrivance shown in fig. 139. Here the micrometer 

 m is raised or lowered by turning the cap a, which is connected with a 

 piece g having a thread cut in it, and by this means e with the 

 micrometer is raised or lowered in a similar manner to the arrange- 

 ment for correction-adjustment in objectives. 



Herr E. Winkel overlooked the fact, however, that in getting rid 

 of the objection to any movement of the eye-lens he had introduced a 

 similar cause of error. Any movement of the micrometer shifts it 

 from the plane of the image, and to bring the latter into coincidence 

 again it is necessary to refocus the objective, and this alters the 

 magnifying power. 



Method of Webbing the Filar Micrometer-! — Mr. D. Gill gives 

 the following directions for webbing a micrometer. 



A spider (the variety is marked by a cross on the back, and is 

 found in English gardens about decayed wood) is caught, and placed 

 on a wire fork. The insect immediately attaches a web to the wire 

 and begins to lower itself by the web to the ground. This web is 

 wound up on the fork till ten or twelve turns, separated by a con- 

 venient space, have been secured. A brush with varnish is then 

 passed along the prongs ; the webs are thus securely fixed to the 

 fork. The parallel prongs of the fork must be sufficiently far apart 

 to allow the web-frame of the micrometer to pass between them. 

 The frame to be webbed is placed on a flat dull black surface between 

 the prongs of the fork, the latter being carefully arranged so that one 

 of the webs lies nearly in the furrow ruled in the frame for its recep- 

 tion. As the web-frame is generally thicker than the fork, the web 

 will now be stretched across the former, with a certain amount of 

 tension, and is brought into the furrow with a finely pointed piece 

 of soft wood. If the surface of the frame is well polished, and the 

 furrows sharply cut without " burr," the web should leap sharply and 

 decidedly into its place. Each end of the web is then secured by a 

 drop of shellac varnish, which should be allowed to harden 

 thoroughly before the frame is touched. The webs can be very 

 readily so handled against a black background, with the aid of a 

 hand lens of two or three inches focus. In experienced hands this 

 method gives good results, but the following, which is generally 

 followed on the Continent, is preferable. 



A web about two inches longer than the width of the frame, is 

 unwound from a cocoon,f and small pieces of lead are attached to its 

 extremities by beeswax. One end of the web, with its attached lead, 

 is laid on a piece of cork floating in a tumbler of water ; the other 



* Zeitschr. f. Wiss. Mikr., ii. (1885) pp. 41-3 (2 figaA Cf. Zeitschr. f 

 Instrumentenk., v. (1885) p. 326. 



t Encyclopaedia Britannica, 9th ed., xvi. (18S3) p. 24S. 



J It is asserted that webs from cocoons are more elastic, better shaped, and 

 more durable than those obtained during an effort of the insect to escape. The 

 best webs we have seen were from a cocoon obtained in Holland, but we have 

 been unable to ascertain the name of the species of spider. 



