INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 327 



impossible to judge with any degree of certainty as to tLe actinic 

 power of tlie light forming the image on the screen by merely looking 

 at it, and that a trial plate only will give an idea as to the length of 

 exposme of the plate for a given day, time of day, and subject to be 

 photographed. It is true we can expose a dry plate for trial, but then 

 we must develop it immediately, and the time it takes to develop a 

 dry plate is about three times that of develoinng a wet plate, and a 

 dry plate is also about thi'ee times as costly as a wet one. Therefoi'e 

 the old wet process is, he considers, the best. 



Micrometre or Micromillimetre.* — Dr. J. Phin points out that 

 about ten or fifteen years ago the prefix micro came into general use in 

 English in certain departments of physics denoting the millionth part 

 of the unit, and about five years ago the British Association adopted 

 this term, which was embodied in the report of the Committee on 

 Dynamical and Electrical Units. This rej)ort, which includes the 

 relations of weights and measures to all measm-ablc quantities, has 

 been generally adopted by the scientific men of Great Britain and 

 America. It will therefore, he thinks, be a very unfortunate thing if 

 microscopists retain the word " micromillimetre " for the thousandth of 

 a millimetre, as that special use of the word micro will give rise to 

 endless confusion and misunderstanding, and will tend to separate 

 microscopists fi'om the rest of the scientific world. 



As the author concludes by saying that he " cannot see the ad- 

 vantage which is claimed for such a term, as the expression two 

 micromillimetres is not more intelligible than -002 millimetres," we 

 gather that the remedy he proposes is to drop any special designation 

 for a division of a millimetre. The same reasoning on wliich this 

 conclusion is based leads us, however, to recommend the adoption of 

 " micrometre " instead of " micromillimetre," wliich would secm-e the 

 uniformity desired, besides being a more convenient word. 



Museum Microscope.— This Microscope, which has not previously 

 been figured, is shown in Fig. 16. 



It consists of a brass drum A B, 20 inches long, and 14 inches iu 

 diameter, in the interior of wliich are six hollow cylinders, one of 

 which is shown in Fig. 17. Each of these cylinders can be rotated 

 independently by means of the milled heads shown at I, and traverse as 

 they rotate, so that each of the apertures with which they are pierced, 

 and which are arranged spirally, are successively brought under the 

 Microscope body C. 



The latter can be monocular or binocular, or better still a double 

 or treble-bodied tube, so that two or three persons can sec the object 

 at the same time. The adjustment for focus is made in the ordinary 

 way by the milled liead F, and rectauguhu' movements are given to 

 tlie body by the four milled heads at E E, so as to bring any desired 

 part of the object into the centre of the tield of view. 



Two additional objectives of difierent powers can be used by 

 moving the catch G to the right or left of the central point. 



Illumination of tho objects is effected by means of the minor D, 



* 'Am Juiirn. IMicr.,' v. (IbSO) i>. l;». 



