The Use of Low Powers with Beep Eye-Pieces. 169 



THE USE OF LOW POWERS WITH DEEP EYE- 



PIECES. 



BY HENRY J, SLACK, F.G.S., 



Member of the Microscopical Society of London. 



If I may judge from the practice of those microscopists with 

 whom I am personally acquainted, and from the best works on 

 the subject of microscopic manipulation, it would appear that 

 sufficient attention has not been paid to the circumstances under 

 which magnification is best obtained by the employment of low 

 objectives and deep eye-pieces; nor has the power of eye- 

 pieces been as yet brought up to the highest point of practical 

 usefulness by microscope-makers. 



In the Micrographic Dictionary all that I notice upon eye- 

 pieces occurs in the introduction, at the bottom of p. xvii. 

 (second edition), and runs as follows: — " Always one, and 

 sometimes two eye-pieces are obtained with the microscope ; 

 but the highest eye-piece which is made should always be pro- 

 cured, for although high eye-pieces are so far objectionable that 

 they magnify the imperfections formed by the object-glass as 

 well as the image itself, yet they frequently render parts of 

 structure distinct which are perhaps only just perceptible with 

 a lower eye-piece." This passage shows that the able authors 

 of the Micrographic Dictionary merely regarded an increase in 

 the power of the eye-piece as a substitute, of a make-shift kind, 

 for an augmentation in the power of the objective. In the 

 valuable lectures of Mr. Lionel Beale, entitled, Blow to Work 

 with the Microscope, the eye-piece question is only glanced at, 

 and in the last edition of Dr. Carpenter's Microscope and its 

 Revelations, which is universally regarded as the best authority 

 on the practical use of the instrument, we are told that the 

 utility of deep eye-pieces will mainly depend upon the excel- 

 lence of the . objectives, and that the best objectives, by per- 

 mitting the use of deep eye-pieces, with little loss of light or 

 definition, afford us a clear gain in range of power, if compared 

 with inferior productions that will only work with low eye- 

 pieces, through not being able to stand the magnification of 

 their defects. 



These remarks are true and valuable ; but, like those in 

 the Micrographic Dictionary, they omit the consideration of the 

 advantages which low objectives and deep eye-pieces have, for 

 certain inquiries, over higher objectives and eye-pieces of less 

 power, and it is to this point that I wish to draw the attention 

 of your numerous microscopic readers. 



Every one knows that the penetration of low object-glasses 



