ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 687 



correction-collar the draw-tube should be pushed in if the object appears 

 too green, or if too pink drawn out until the ruby tint is obtained, 

 assuming, that is, that the objective corrects in that colour. 



In the above note we remark that Professor Moore does not specify 

 whether the y 1 ^ objective was dry or immersion. It should also be 

 observed that in testing the colour-corrections of a large-apertured 

 immersion objective on a dry Podura adhering to the cover-glass, it 

 may happen that there is an appreciable film of air between the scale 

 and the surface to which it adheres, in which case the " ruby " tint 

 may be replaced by a deep red colour which cannot be corrected by 

 the adjustment collar. The objective will then be acting as a badly 

 corrected dry lens. In such a case a scale must be sought that is 

 more closely adherent to the cover-glass. 



It is a fact well known to opticians that objectives of large aper- 

 ture which are very perfectly achromatized do not yield such sharp 

 definition of a dry Podura-scalo as those in which the outstanding 

 colour-aberration is of a moderate ruby tint. The more closely 

 adherent the scale is to the cover-glass, the less red should be the 

 tint ; and if by means of the vertical illuminator or equivalent 

 means a scale is chosen which adheres closely, the ruby tint will be 

 less pronounced, and the definition generally more perfect. 



Measuring Thickness of Cover-glass by Correction Collar.* — 

 Professor 0. K. Wead points out that the thickness of a cover-glass 

 " may be found quite closely by means of an objective with correction. 

 Taking the covers used above ["0058 inch and • 0123 inch], and having 

 focussed on dust or finger marks on the under side, turn the collar 

 till dust on the upper side is in focus ; with the thinner glass several 

 trials gave as the reading of the collar 3° -6, 3°*75, &c. ; working 

 backwards focussing on the top with the collar at 9°*6 and then on 

 the lower side by the collar the reading was 6° ■ 1 twice, a change of 

 3*5; mean of seven trials gave 3 0, 56; similarly with the thicker 

 cover, mean of five trials gave 7° # 58. If we assume the change of 

 the collar to be just proportional to the thickness of the glass, since 

 the thin glass is" -0058 inch we should have 3-56 : 7*58 : : -0058 : 

 thickness of thick cover: solving we find it to be • 01235 inch — a 

 difference of less than T0 Vtt ^ nc ^ from that found by a Brown and 

 Sharpe's gauge — a quantity scarcely measurable with this gauge. If 

 one has, then, a single cover-glass whose thickness is known, by a 

 simple proportion the thickness of any other one can be found in a 

 moment. For this particular lens the reading of a collar multiplied 

 by 1 • 6 will give very closely the thickness in thousandths of an inch. 

 Makers might easily furnish for their lenses the constant multiplier 

 to be used as this 1 • 6 is ; or divide the scale so as to indicate directly 

 the thickness in thousandths of an inch." 



Bausch and Lomb Optical Co.'s Glass Stage and Slide- 

 carrier.f — This (Fig. 128, see also Fig. 113) is intended as a sub- 

 stitute for the mechanical stage to a certain extent. It consists of a 



* ' The Microscope,' ii. (1882) p. 72. 



f From the Company's Price List, 7th ed., 1882, p. 5 (1 fig.). 



3 a 2 



