248 T. F. SMITH ON ARACHNOIDISCUS. 



and, with the help of the makers, I was enabled to have them put 

 right, and they then performed as brilliantly as my own. 



Cheap oil-immersion object-glasses, in fixed settings, are now 

 turned out by the score, both here and on the Continent, and as it 

 is not possible to alter the corrections except with the draw-tube, 

 it is of the utmost importance that the lenses should be set at a 

 point to work at their best on the vast majority of objects. 



This is only reached in the oil-immersion when the object is 

 mounted in a medium of the same refractive index as glass, and 

 hence the mischief of setting it on a test-object, mounted dry, and 

 supposed to be on the cover, but which is not. 



Whatever may be the difference of opinion as to the structure of 

 the Podura scale, there is no difference of opinion about the appear- 

 ance of certain conventional markings on it, and that a glass which 

 will show these markings well will work well on every other object. 

 This may be taken for granted, and if the glass has a correction 

 collar the observer can adjust for himself ; but if there is no such 

 means of adjustment he is reduced to do what he can with the 

 draw -tube, which, in my experience, would require to be nine 

 inches long to make a glass, set on the Podura scale, work well on 

 an object mounted in balsam. 



1 need not say that the increase of the size of the image, and 

 the loss of light, would make it impossible to satisfactorily adjust 

 that way. 



There are two great objections to using the Podura scale as a 

 test object for an oil-immersion. The first is, that the conventional 

 markings can only be seen when the scale is a little way off the 

 cover-glass, and, consequently, the objective not working at its full 

 aperture ; and, secondly, it is impossible to tell the best point. 



A dry glass, on the Podura scale, is exceedingly sensitive, and a 

 little turn of the correction collar, or a little difference in the length 

 of the draw-tube, will make all the difference between fine defini- 

 tion and no definition at all. With the oil-immersion, however, 

 you can go through the whole range of the correction collar with- 

 out making any difference in the markings, beyond changing them 

 from red to blue. Of course, opticians will tell you that they know 

 the best point, but my experience is as follows : — 



Four object-glasses, with a correction collar, were supposed to 

 be set with best definition on the Podura scale at the point ; the 

 first (my own) is best on a balsam-mounted slide at point 1\ ; No. 



