704 SUMMARY OF CURRENT RESEARCHES RELATING TO 



socket for the sliding body-tube. This latter application of the drum 

 has no raison-d'etre that we can discover, adding nothing to the con- 

 venience or stability of the instrument. 



Theiler's " Universal (Achromatic) Pocket Microscope." — The 



commendations of Microscopes and microscopic apparatus by what we 

 may term the " lay " press are often very wonderful, and after reading 

 the following descriptions it was not perhaps surprising that we . 

 should have become somewhat eager to possess the instrument. 



" We have received from Messrs. Theiler and Sons a specimen of 

 their Universal Pocket Microscope, which magnifies 50 diameters. It 

 is a very admirable contrivance, and should 

 Fig. 144. be in the hands of all young people." 



" This instrument is a virtual Micro- 

 scope, giving beautifully well-defined images, 

 and may be used either by the aid of day- 

 or lamp-light. No one having any interest 

 in microscopy should leave home unaccom- 

 panied by such a small and so efficient an 

 instrument. Its cost may be measured in 

 the inverse ratio to its utility and value." 



The instrument when received (from 

 Messrs. M. Theiler and Sons, of London) 

 turned out to be the familiar " Taschen-Mikroskop " of the German 

 opticians supplied for many years past. It is shown in fig. 144. 

 The slide is inserted in the slit of the tube by pressing down the 

 spring which keeps it in place. The adjustment of focus is effected 

 by screwing the lens in or out. Some of the German makers supply 

 the instrument to take ordinary 3x1 slides. 



Eye-piece Micrometers.* — Mr. H. L. Tolman records that for 

 some months past he and Dr. M. D. Ewell, having been working at 

 micrometry and the relative r-^L vantages of the eye-piece and cobweb 

 micrometers, decided to make a series of independent measurements 

 to see which method was superior. 



Two slides of fresh blood were prepared under the same circum- 

 stances, as nearly as possible, the blood was dried about half an hour 

 in the air of a well-warmed room, and then sealed in a cell, so that 

 the degree of desiccation would be the same, and the measurements 

 were made the same evening, independently. Dr. Ewell used a 

 1/10 Spencer (homogeneous immersion, N.A. 1"35) with an amplifier 

 and a 1 in. eye-piece, giving a power of about 2000, and Mr. Tolman 

 a 1/10 Spencer (homogeneous immersion, N.A. 1 • 25) with a 3/4 in. 

 eye-piece, power 1562. The former measured twenty-five corpuscles, 

 the average being 1/3138 in., and the latter measured fifty with an 

 average of 1/3139 in., the difference between the measurements being 

 only 1/985,000, an amount far too little to measure. Mr. Tolman 

 feels pretty well convinced, therefore, that the cobweb micrometer 

 does not offer sufficient advantage in point of accuracy to compensate 

 for its additional cumbersomeness and expensiveness. 



* Amer. Mon. Micr. Joum., vi. (1885) pp. 115-6. 



