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



pushing the power of a low objective be carried much beyond 

 five times the result of the first eye-piece, a totally different 

 construction must be resorted to, or the field will be - inconve- 

 niently restricted and the light too weak. What may be the 

 best possible form I leave to those specially skilled in optics, 

 and to our great microscope makers ; but there is an admirable 

 telescope eye-piece made by Steinheil, of Munich, which works 

 well with a three-inch at ten times the power of the first 

 microscope eye-piece, or at much more if the draw-tube be 

 employed. With this eye-piece and my three-inch I obtain 

 highly advantageous views of small irregular zoophyte cells, and 

 of some of the larger diatoms, whose parts^ like those of the Cam- 

 pylodiscus sjyiralis, lie in such different planes as not to come 

 into focus at once if half- inch or higher objectives are employed. 

 Some of the uneven circular diatoms exhibit their concavities 

 and convexities in a surprising manner by this treatment and 

 dark ground illumination. Of course there is a sacrifice of the 

 minute details that cannot be seen without a higher angle of 

 aperture, which must be resorted to for that purpose ; but the 

 gain, and a very important one for special inquiries, is in the 

 'penetration, which is sufficient to bring all parts into simulta- 

 neous view. 



The eye-piece used in these experiments was sent to me by 

 Steinheil as being the best — indeed the only one, he said, would 

 be satisfactory as regarded light and field — to obtain a power of 

 300 with an excellent 42-inch telescope, having a three-inch 

 object glass (made by him) upon Gauss's plan. Used as a 

 microscope eye-piece it is nearly as light as a much lower Huy- 

 ghenian one, and with the three-inch objective takes in a field 

 one-twentieth of an inch in diameter, with a magnification of 150. 

 A power of 130 obtained with a two-thirds, second eye-piece, 

 and two and-a-half inches of draw-tube, had the same field. It 

 is called in Steinheil' s catalogue MikrosTcop als Ocular, and is an 

 erecting combination composed of three plano-convex lenses 

 and a compound front lens. I have used it in extreme ways — 

 with the three- inch power, and with Smith and Beck's one- 

 twentieth, when the enlargement is enormous, and might be 

 advantageous for special inquiries. It is however Avith very 

 low objectives that I think very high eye-pieces maybe most 

 usefully employed, and if the attention of raicroscopists is 

 directed to 1 1 > < • question many objects will become visible as 

 wholes, which have hitherto only been looked at in a piecemeal 

 way. 



Small crystals, liko those of arsenic, which have polyhedral 

 figures, cannot, be well shown if such a power us L50 to 200 be 

 obtained in the usual way. 1 1" you focus the tops of the pyra- 

 mids, the bottoms are out of sight, and vice versa; but all 



