1890.] MIOKOSCOPICAL JOUKNAL. 29 



recognized, whether or not we admit others. In the first place, these 

 tubes permit the use of diaphragms to cut off the injurious reflection from 

 the sides. It is impossible to blacken the inner surface of the tube so 

 that there shall be no reflected light, and the usual attempts at this are 

 dangerous to objectives, because the crumbling particles fall on to the 

 back lens to an exceedingly injurious extent. Hundreds of workers 

 over the tube have no conception of the state of their objectives in this 

 particular. If they did it would often be so much the worse for the 

 objectives, for the attempts to clean them would frequently injure more 

 than cure. With metal diaphragms, properly placed and shaped, the 

 unused light can be thoroughly intercepted. At the same time a safe- 

 guard instead of a danger is provided for the optical parts. Again, 

 larger tubes make it possible to use larger field lenses with low-power 

 eye-pieces, a thing of no mean importance. This, however, depends 

 somewhat upon the length, because the longer the tube the wider the 

 upper end may be with advantage. 



In regard to the length, I cannot but feel that too little attention is 

 usually given by those who use the instrument to the proper adjustment 

 for the objectives employed. If these are without collar corrections 

 and are made for a ten-inch tube with covei'-glass of a definite thick- 

 ness, then the best work cannot be expected of them with any other length, 

 if the same kind of eye-piece is used. Whatever else is sacrificed this 

 best work should always be secured. There is nothing that can com- 

 pensate for impairment of the best possible image from the objective. 

 The microscopist must know what that best image is, and must know^ 

 how to secure it or he fails in an important element. The special 

 length of tube for the special objective becomes, therefore, a practical 

 consideration of transcendent import. Draw-tubes are of great service 

 if intelligently managed, but otherwise may easily be the source of 

 much unsuspected mischief. It is well known that English and Amer- 

 ican objectives, as well as some others, are constructed for a ten-inch 

 tube. For these, under usual circumstances, there is no alternative as 

 to length of tube for the stand, unless, indeed, an eye-piece is especially 

 adapted to the changed conditions . Beginners especially ought to rec- 

 ognize this imperative condition of things. Of course the same reason- 

 ing holds good in regard to objectives and tubes of shorter working. 

 With an objective constructed for a tube length of one hundred and 

 sixty millimeters this latter length should be used and no other, except 

 as modifications are made by collar adjustment, thickness of cover, pe- 

 culiarity of eye-piece, and the like. 



So far there is no choice offered ; but if it is a question of convenience 

 or of merit between a six-inch and a ten-inch tube, before purchasing 

 either stands or objectives, then there is a chance for selection. So far 

 as the optical question is concerned, there can be no great diflerence, 

 or we should have long ago heard more of it from those who have 

 made these matters a special study. Of course the curvatures of the 

 lenses must be increased for the short tube to furnish the same magfni- 

 fication. In other words, those who work with short tubes must use 

 objectives or eye-pieces of somewhat greater power to secure the en- 

 largement obtained with the longer combination. In my experience 

 the higher the power of the eye-piece the more fatigue to the eye in 

 prolonged work, and certainly the higher the power of the objective 



