348 THE AMERICAN MONTHLY [Nov 



and does not lose its normal symmetry. The cotton pro- 

 tects the convolutions from being- ruptured. 



Holoscopic Eyepiece.— Many workers have in their bat- 

 tery of objectives samples of both the achromatic and apo- 

 chromatic types,having- their favorites of both series. But 

 this has necessitated two sets of eye-pieces, the Huyg-hen- 

 ian for the former, and the Compensating- for the latter ; 

 but this will no long-er be necessary, because in the "Holo- 

 scopic" eye-piece of W. Watson & Sons we have an eye- 

 piece which has been specially devised for both purposes. 

 It consists of two tubes, the outer one carrying- the field 

 lens, and the inner, or telescopic one, having- fixed to it the 

 eye lens and diaphrag-m. Carefully selected appropriate 

 g-lasses are used for the lenses, and when the telescopic 

 tube is pushed in as far as it will g-o, the eye-piece is an un- 

 der-corrected one, suitable for work with achromatic ob- 

 jectives. As the tube is drawn out, the eye-piece becomes 

 increasing-ly a compensating- one, possessing- the so-called 

 over-correctionassociated with the compensating-eye-pieces 

 which are used with the apochromatic objectives. This 

 ey^-piece yields really beautiful imag-es, and not only does 

 it obviate the necessity for two sets of eye-pieces, but it 

 g-ives to the worker a power of adaptation which he has not 

 hitherto possessed. Very few of the apochromatic objec- 

 tives have the same amount of under-correction, conse- 

 quently with the fixed eye-pieces the over-correction is 

 frequently too g-reat. With the "Holoscopic" eye-piece, the 

 over-correction can be exactly adjusted to the objective, 

 and a divided scale is eng-raved upon the draw-tube, so that 

 the position may be recorded. It is made in the following^ 

 magnifying- powers : — 



For the six-inch tube leng-th, 5, 7, 10, and 14 diameters. 



For the ten-inch tube length, 7, 10, 14, and 20 ,, 

 The cost is very little g-reater than that of the Huyg-hen- 

 ian eye-pieces. — Knowledge. 



