INVEKTEBEATA, CRTPTOGAMIA, MICROSCOPY, ETC. 701 



If the slides are generally of the character of those exhibited the 

 supi)ly must, we are afraid, for some time fall short of the demand, as 

 there will be few biologists who will not desire to add some of the 

 slides to their cabinet. 



Homogeneous-Immersion Lenses.* — Mr. A. A. Biagdon, referring 

 to the strong impression prevalent among microscoj^ists that objectives 

 having high interior angles, say 90° and upwards, are of no use 

 except to amuse diatomists, says that this is by no means the true 

 state of the case. On comparing the definition obtained with a water- 

 immersion objective of 105° interior angle (by Tolles) with other 

 lenses having 120° or 140° air angle, the image with the latter was 

 shown to be imsatisfactory. And again on comparing the water- 

 immersion with the same maker's recent homogeneous-immersion 

 having 127° interior angle, the advantage was decidedly with the 

 latter. He refers to the series of microphotographs by Dr. J. J. 

 Woodward f of A. pellucida mounted in balsam, with Zeiss's ^^^ and \ 

 oil-immersions, together with other notable objectives for comparison 

 of their respective merits. Among these lenses were a i and -^jj inch 

 by Spencer, glycerine-immersion, and a -jig -inch oil-immersion by 

 Tolles, and says that " it is only necessary for any unprejudiced 

 person to examine this series of photographs to decide at once as to 

 the superiority of the homogeneous-immersion lenses in defining 

 power." 



Mr. Bragdon approves of Mr. Tolles retaining the screw-collar 

 with homogeneous-immersion lenses for these reasons, — that it affords 

 a means of using water as an immersion medium when several 

 preparations are being mounted of one kind, and it is desired to make 

 a cursory examination of them at once with high powers before any 

 change shall have taken place, and without waiting for covers to 

 become fixed by hardening of the balsam ; the collar-adjustment is 

 also useful, even with the homogeneous-immersion, to obtain the best 

 image with diifercnt lengths of draw-tube. 



Fluid for Homogeneous Immersion. J — Mr. Bragdon finds that the 

 best medium for homogeneous immersion is glycerine brought up to 

 the required index by making a saturated solution witli it and sulj)ho- 

 carbolatc of zinc : there is only one, and that not a serious, objection 

 to its every-day use, viz. that it is just a little too thick. 



Dr. Blackham also says § that, " good heavy glycerine is the best 

 immersion medium he has found out of many ; it docs not evaporate, 

 soften cement used in mounting objects, nor smell badly, is not 

 poisonous nor irritant, and is in every way satisfactory." 



Errors of Refraction in the Eyes of Microscopistsll— Dr. J. C. 

 IVIorgan points out tliat the requirements in construction and adjust- 

 ment of glasses and the results of work done must vary greatly with 



* ' Am. M. Micr. Journ.,' i. (1880) pp. 89-93. 

 t Poo this .Tournnl, ii. (IHTK) p. C,~2. 

 X 'Am. M. l\Iicr. Jotirii.,' i. (ISSO) p. 92. 

 § ' Eu<i\. Mooh..' xxxi. (IS.SO) p. 100. 

 il 'Am. Juurii. Micr.,' v. (IS^O) p. 91. 



