INVERTEBRATA, CRYPT0GA3IIA, MICROSCOPY, ETC. 1051 



follows then that a reduction of the aperture by making the front 

 lens thinner immediately provides greater working distance without 

 affecting the aberrations, for as the first refraction takes place at the 

 posterior (curved) surface of the front lens the removal of any portion 

 of thickness at the auterior (plane) surface simply cuts off zones of 

 peripheral rays without altering the distance — the distance being at 

 once filled up by the homogeneous-immersion fluid or by an extra 

 thickness of covering-glass. An extra front lens may then be applied 

 to the back combinations of such a ^^^ to enable the observer to view 

 an object through a covering-glass that would be practically a maxi- 

 mum thickness for an ■} (aperture = 90^) constructed on the usual 

 formula where the setting encroaches on the active spherical refrac- 

 ting surface ; a second front might give a high average aperture for 

 a -Y^ (115'^), whilst the thickest front (representing the maximum 

 aperture of the whole construction, 142') enables the observer to view 

 an object with a greater aperture than has hitherto been obtained with 

 any j\-., owing to the difficulties of construction, and through a thicker 

 covering -glass than a y'.y of this aperture (even if it could bo success- 

 fully made) would permit of ; hence the three different fronts would 

 give a great range of aperture with a corresjionding range of working 

 distance, which is practically what is sought by having objectives 

 constructed of the three different foci, i, yV, and y\r. 



We imderstand from Messrs. Powell and Lcaland that for an 

 aperture of 115° in glass, there would be no necessity to mount 

 the front lens on a plate, that aperture having already been success- 

 fully obtained and exceeded by mounting the front in the usual 

 way. The purpose of the plate (which is only '003 in. thick) is, as 

 before mentioned,* to allow of a portion of the posterior curved 

 refracting surface of tho front lens beyond the hemisphere to be 

 utilized. 



Fluid for Homogeneous-immersion Objectives.— Mr. A. A. Brag- 

 don writes to us in regard to his note inserted at page 701. After 

 referring to tlic fact that sulpho-carbolate (jf zinc was first suggested by 

 Professor Abbe,t he says that cedarwood oil, in his opinion, can never 

 become generally useful. It varies so much in diftcront samples that 

 even an index of 1*512 as first named for it cannot be relied upon. Then 

 it is so fluid that it runs all over slide and stand, so that the objective 

 cannot be immersed without placing the Microscope erect every time. 

 Experimenting with it, however, in combination with other oils, 

 Bonie good results were obtained, e. g. with oil of anise, although not 

 equal to the zinc and glycerine, which can be as easily cleaned from 

 the slide and kns as glycerine by using water. 



By taking equal -parts by weight of C. P. glycerine (Price's) and 

 Bulpho-carbolate of zinc crystals, mingling the two, and applying 

 heat sufficient to lujil the glycerine, a solution of proper index wvn bo 

 obtained for use with a Zeiss objictive of 1*50 index, or a Tolles of 

 1 -525 index (i. e. for all practical purjxises). If, however, one desires 

 to be exact for the latter, the solution will have to be evapoi*ated 



• See this Jourmil, mdr, pp. 884 5. t "'i'l- ii- (l'^T:») j.i.. 'MC, and 82:^. 



