872 A New Reversible Compressorium. 



the firm of Smith, Beck, and Beck, but it is more complete 

 than that suggested by Dr. Carpenter. It consists essentially 

 of a very flat screw live box, having a special contrivance to pre- 

 vent the rotation of the compressing plate, and furnished with 

 springs which elevate the upper plate the instant the tightening 

 of the screw is relaxed. It is so thin that it offers no obstacle 

 to the employment of the achromatic condenser or the para- 

 bolic illuminator, and requires no change in the adjustment of 

 the illuminating apparatus when it is reversed. 



Another important peculiarity in this compressorium is the 

 way in which the upper and lower glasses are inserted, and the 

 adaptation of the instrument to receive them either moderately 

 thick or extremely thin. The glasses are kept in their places 

 by a slight pressure of four flat-headed screws, two for each. 

 These screws can be instantly turned by a small pair of pliers, 

 or by the finger and thumb, and allow a glass to be replaced or 

 exchanged without a minute's delay. This we regard as a 

 point of great practical value, as much time is lost when the 

 glasses are cemented in. In addition to replacing broken 

 glasses it is frequently desirable to change them, as the very 

 thin glass necessary for high powers will not bear the strain 

 that is inevitable, when many objects that are intended to be 

 examined with lower powers have to be compressed. With the 

 new compressorium thicker plates, top and bottom, can be sub- 

 stituted for thin, or vice versa, at a moment's notice, and 

 Messrs. Smith and Beck can, if required, supply two or more of 

 the compressing apparatus to fit the same bed, and thus two or 

 more thicknesses of glass may be always ready for use. 



The new compressorium was shown at the October meeting 

 of the Microscopical Society of London, and met with appro- 

 bation from those who examined it ; but we purposely delayed 

 noticing it to see if we could discover any practical defect. We 

 have now employed it on many occasions with frequent change 

 of glasses and with high and low powers, and it appears to us 

 to fulfil more completely than any other compressorium we are 

 acquainted, the varied requirements with which such an in- 

 strument is usually employed. In the details of its arrange- 

 ment there arc many little pieces of clever contrivance that 

 would not bo intelligible without diagrams, but we have ex- 

 plained the main features of the instrument, which is simple, 

 convenient, and strong. 



While upon the subject of compressoriums we may remark 

 that, when the parallelism of the two glasses is required to bo 

 extremely perfect, us in the case of minute and very flat 

 objects, cither side of which cannot be viewed with deep 

 objectives, (-.we is required in the selection of thr glasses, as 

 the ordinary thin glass made for covering microscopic objects is 



