18 Transactions of the Society. 



phragms, &c. being added to complete the efficiency of the appara- 

 tus as a whole. 



It must be here remarked that for the full efficiency of this 

 arrangement to be secured it is absolutely necessary to obtain 

 critical light most carefully. For this reason it may be well to 

 mention how this may be most easily accomplished. 



Having removed the film-prism and its saddle from the optical 

 bench, the collimating lens is covered with a slip of black glass, 

 shown in the diagram, so that the eye ma3 T not be blinded. This 

 should not be forgotten, for the direct light is most intense. Owing 

 to most condensers being made to work with more or less diverging 

 beams, such as those derived from a lamp, and not with parallel 

 rays such as in this case issue from the collimator, it is in most 

 cases necessary to raise the condenser nearer the specimen than 

 would be the case if a lamp at a few inches were used. In conse- 

 quence, especially if the slip be a thick one, it may in some in- 

 stances be impossible to raise the condenser high enough to see the 

 markings on the lime — in other words, to obtain critical light. To do 

 this properly, then, a 7 to 10-in. common negative spectacle lens must 

 lie placed immediately beneath the condenser, which rectifies the 

 trouble and enables the microscopist to obtain the critical images 

 without further difficulty. In my case, Mr. Conrady computed and 

 made me a special front to his condenser, which does away with the 

 necessity. With respect to the selection of an immersion con- 

 denser, it is most important to obtain one with as large an aplanatic 

 cone as possible, because, failing this, less oblique light can be 

 profitably used, and the final resolution is affected most seriously. 

 It should be recollected, too, that as the spectra from the hori- 

 zontal as well as the transverse lines must both at one and the 

 same time be seen in the back lens of the objective (the eye-piece 

 being removed), it is obviously most important to use one with as 

 high a N.A. as possible, for a great difference is noticeable in the 

 resolving power even if a 1 • 30 apochromatic be used instead of 

 a 1-40. 



Seeing that one of the great difficulties hitherto experienced in 

 using monochromatic light with a mixed jet is to get enough of it, 

 so it is only fair to point out that the best limes procurable should 

 be used, but perhaps it is more important still to obtain the purest 

 •oxygen. Some oxygen on the market contains a large amount of 

 impurities, chlorine, air, and so on, probably through carelessness 

 in its manufacture ; hence, while admitting that others may make 

 as good, the author has no hesitation in recommending the gas 

 produced by Brin's Oxygen Co., for the simple reason that, owing 

 to the company frequently testing it during manufacture in every 

 process, and finally before filling the cylinders, it is sent out prac- 

 tically pure. 



