CH. //] LABORATORY MICROSCOPES 65 



as follows : "Whitworth thread, i. e., a V shaped thread, sides of thread inclined 

 to angle of 55 to each other, one-sixth of the V depth of the thread being 

 rounded off at the top of the thread, and one-sixth of the thread being rounded off 

 at the bottom of the thread. Pitch of screw, 36 to the inch ; length of thread on 

 object-glass, 0.125 inch ; plain fitting above thread of object-glass, 0.15 inch long, 

 to be about the size of the bottom of male thread ; length of thread of nose-piece 

 [on the lower end of the tube of the microscope] , not less than o. 25 inch ; diam- 

 eter of the object-glass screw at the bottom of the screw, 0.7626 inch ; diameter 

 of the nose-piece screw at the bottom of the thread, 0.8 inch." 



In order to facilitate the introduction of this universal screw, or as it soon 

 came to be called " The Society Screw," the Royal Microscopical Society undertook 

 to supply standard taps. From the mechanical difficulty in making these taps 

 perfect there soon came to be considerable difference in the "Society Screws, " and 

 the object of the society in providing a universal screw was partly defeated. 



In 1884 the American Microscopical Society appointed Mr. Edward Bausch 

 and Prof. William A. Rogers upon a committee to correspond with the Royal 

 Microscopical Society, with a view to perfecting the standard "Society Screw," 

 or of adopting another standard and of perfecting methods by which the screws of 

 all makers might be truly uniform. Although this matter was earnestly consid- 

 ered at the time by the Royal Microscopical Society, the mechanical difficulties 

 were so great that the improvements were abandoned. 



Fortunately, however, during the year (1896) that society again took hold of 

 the matter in earnest, and the "Society Screw" is now accurate, and facilities for 

 obtaining the standard are so good that there is a reasonable certainty that the 

 universal screw for microscopic objectives may be realized. It is astonishing to 

 see how widely the "Society Screw" has been adopted. Indeed there is not a 

 maker of first-class microscopes in the world who does not supply the objectives 

 and stands with the "Society Screw, " and an objective in England or America which 

 does not have this screw should be looked upon with suspicion. That is, it is 

 either old, cheap, or not the product of one of the great opticians. For the Stand- 

 ard, or "Society Screw," see : Trans. Roy. Micr. Soc. , 1857, pp. 39-41 ; 1859, pp. 

 92-97 ; 1860, pp. 103-104. (All to be found in Quar. Jour. Micr. Sci., o. s., vols. 

 VI, VII and VIII). Proc. Amer. Micr. Soc. 1884, p. 274; 1886, p. 199 ; 1893, p. 

 38. Journal of the Royal Microscopical Society, August, 1896. 



In this last paper of four pages the matter is very carefully gone over and full 

 specifications of the new screw given. It conforms almost exactly with the orig- 

 inal standard adopted by the society, but means have been devised by which it 

 may be kept standard. 



124. Standard Size Oculars and Substage Condensers. For a considera- 

 tion of these, with measurements, see f 46, 87. 



MARKERS AND MECHANICAL STAGES 



Markers are devices to facilitate the finding of some object or part which it is 

 especially desired to refer to again or to demonstrate to a class. The mechanical 

 stage makes it much easier to follow out a series of objects, to move the slide 

 when using high powers, and for complete exploration of a preparation. Most of 

 the mechanical stages have scales or scales and verniers by which an object once 

 recorded may be readily found again. 



