306 The Microscope. 



must be supplied. If the numbers thus obtained be compared 

 with the known real thickness of the covers, a reduction factor 

 is obtained from their quotients, which is available for deter- 

 mining measurements of a similar kind, that is to say, for 

 measurements of other cover glasses with the same objective, 

 ocular, diaphragm and tube length. The focussing differences 

 are always to be multiplied by this factor in order to obtain the 

 true thickness of the layer. 



As an example — Objective DD Zeiss, diaphragm 8 mm in 

 diameter; tube length 155 mm.; and four cover glasses, the 

 thickness of which, already ascertained, are 0.146, 0.168, 0.187, 

 0.22. The focussing differences marked by the head of the fine 

 adjustment screw were 35, 40, 45, 52 divisions. Then the re- 

 duction factors in 1/1000/^. are VV =4.17; W = 4.20; W = 

 4.16 ; W = 4.23 ; or on the average 4.19, say 4.2. If the thick- 

 ness of these cover glasses had not been known, but the focus- 

 sing difference had been obtained and multiplied by 4.2, the 

 results would have been 0.147, 0.168, 0.189, 0.218, instead of 

 0.146, 0.168, 187, 0.22. Differences of + 0.001, 0.0, -[- 0.002, 

 — 0.002 ; a result more than sufficiently accurate for the purpose. 

 It is also exceedingly important to know the thickness of the 

 cover when using non-adjustable objectives. These lenses are 

 corrected by their makers for a certain thickness, and as the 

 adjustment cannot be changed, and since the microscopist, 

 until recently, has not known for what cover thickness the ob- 

 jectives were intended, he was at the mercy of circumstances, 

 and forced to accept and be content with whatever image he 

 could get. Now we are able to use covers of at least approxi- 

 mate correctness with our non-adjustable objectives, thanks to 

 Prof. S. H. Gage, of Cornell University, who has investigated 

 this matter. Prof Gage submitted certain questions in relation 

 to the subject to the various opticians of the world, and from 

 his published results I take the following, so that the reader, if he 

 possesses non-adjustable objectives l\y any of these makers, may 

 use covers of the proper thickness to obtain the best results, 

 provided his microscope body tube is of the standard length : 

 f J. Grunow, New York. 



2, j H. R. Spenser and Smith, Buffalo, N. Y. 



105 mm. <; ^^^^^ Wales, New York. 



[ Powell & Lealand, London, 

 16-20 mm. Ross & Co., London, 



