The Use of Low Powers with Deep Eye-Pieces. 171 



any power that may "be needed, between 15 to 200 linear, or 

 even more. 



The practical direction should be — when you must have con- 

 siderable resolving power, use a proportionately deep object- 

 glass ■ when you must have great penetrating power, use a low 

 one, and gain your enlargement by the eye-piece, if the object 

 be small. Much more penetrating power is lost in doubling 

 or trebling a given magnification by having recourse to an 

 objective of larger angle of aperture and shorter focal length, 

 than by resorting to an eye-piece of the required strength. 

 The mouth apparatus of moderate sized insects, the cells and 

 appendages of zoophytes and similar objects, in which various 

 parts are in different planes, may be well shown with a three- 

 inch object-glass, and the first, second, or third eye-piece of 

 Smith and Beck's series, when half the parts would be out of 

 focus if a two-thirds object-glass were employed to give the 

 same power. I have, for example, a slide containing that ex- 

 quisite little snailshell, the Helix pulchella, which shows 

 beautifully as an opaque object with about forty-five linear 

 enlargement. Gain this with an inch objective, and part is 

 out of focus ; gain it with a three-inch, and all is taken in. 

 Here again are some Foraminifera, Orbiculina, Glohigerina, etc., 

 and their ins and outs can be displayed better with the three- 

 inch worked up with draw-tube or eye-piece than by any other 

 means. 



The eye-pieces supplied by different makers vary in power. 

 Smith and Beck put three down in their catalogue : the first 

 nearly agreeing with that of other celebrated makers; the 

 second adding three-quarters to the power of the first, making 

 60, for example, into 105; and the third trebling the power of 

 the first. Much higher eye-pieces upon the Huyghenian plan 

 are of restricted use, on account of loss of light and limitation 

 of field; but Boss,* Powell and Lealand (and Smith and 

 Beck, if required) supply eye-pieces which give about five 

 times the power of the A, or No. 1. By such means a 

 three-inch objective, giving 15,f with my first eye-piece, 

 may be raised to 75 with comparatively little loss of pene- 

 tration, and good performance upon any object that will bear 

 the loss of light and limitation of field. It would however be 

 desirable to substitute for the fifth Huyghenian eye-piece an 

 achromatic one like Home and Thornth waiters aplanatic, or a 

 better combination if it could be devised ; but if the principle of 



* Mr. Boss has lately made au F eye-piece, giving about eight times the power 

 of the A. 



t The three-inch employed was by Baker, and estimated to give a power of 17, 

 with his first eye-piece, which is, I believe, somewhat higher than the corresponding 

 power of other makers. 



