382 NOTES. 



powers I cannot conceive anything finer." It also formed a 

 binocular eye-piece and was about the first to be made. In 1881 

 lie introduced another new binocular, in form like Stephenson's. 

 It had parallel tubes, but they were bent as in the Stephenson. 

 The ingenious part of the arrangement was that the beam from 

 the back of the objective was divided by two Wollaston camera- 

 lucida prisms placed back to back ; these deflected the rays right 

 and left, then another prism with two reflections bent them up 

 the tubes. Probably a carefully made binocular on this plan 

 would be a very successful instrument. In 1884 he designed a 

 bent-tube erecting monocular microscope ; obviously a most 

 useful instrument, it is surprising that some energetic manu- 

 facturer has not taken up this idea. 



In the same year he designed a new polarising prism which was 

 further improved by Mr. H. G. Madan in 1885. In 1886 he brought 

 out yet another improved form of polarising prism, the object of 

 which was to lessen the ratio of its length to its breadth. That of 

 the Nicol is about 3:1, while the Ahrens was IJ : 1. In 1887 he 

 made an erecting microscope the design of which has had far- 

 reaching consequences. The erection was obtained byPorro prisms. 

 A description of this instrument, with figure, appeared in the R.M.S. 

 Journal for 1888, p. 1020, fig. 161. He said that " the erection 

 of the image is obtained by two right-angled prisms crossed in the 

 way used in some of the binocular field glasses." This naturally 

 came to the notice of Mr. J. W. Stephenson, the treasurer of the 

 E.M.S., who was especially interested in binoculars and prisms 

 of all sorts. He applied to Ahrens and asked if he could supply 

 him with one of these binocular field glasses. Ahrens replied 

 that they were difficult to make and would cost some £25. Mr. 

 Stephenson thought this too high a price, and wTote to Prof. 

 Abbe, with whom he was in frequent correspondence, to ask if 

 the Zeiss firm would send him a quotation for a pair of these field 

 glasses. This letter of Stephenson set Prof. Abbe on the subject, 

 and 80 was the cause of the introduction of these wonderfully 

 useful glasses. (It is not at all unlikely that Porro got the first 

 idea of erection by two right-angled prisms from Zahn's Oculus 

 Artificialis, a book published in 1702, where there is a figure of a 

 telescope doubled up in a similar manner by right-angled reflecting 

 mirrors). It was Ahrens, however, who first brought the long- 

 forgotten Porroprism to remembrance. The difliculty in the con- 



