ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 221 



(2) Eye-pieces and Objectives. 



Dry and Water Immersion I Objective by Ross. — At the February 

 Meeting R. CI. Hebb exhibited an old lens, the property of the West- 

 minster Hospital (fig. 29). Mr. E. M. Nelson kindly examined the 

 objective, and reports as follows : — 



This ^ is an example of a lens made by Ross upon a formula 

 Mr. Wenham. 



In June 1871, Mr. Tolles, of America, made a^ which 

 could be adjusted by means of its screw collar for either 

 wet or dry use, and Mr. Woodward said that this lens 

 would resolve Amphipleura pellucida. 



When this report was received in England it made a 

 great stir, and opticians were most anxious to equal, if not excel, this 

 result. Hence the origin of the lens. 



It is of a peculiar construction, having a single front and back, and 

 a triple middle. This triple has a central biconcave dense flint lens 

 which corrects the aberration of the entire lens. The plan of this lens 

 is figured on the right-hand half of fig. 6 on page 16:-! of the " Monthly 

 Microscopical Journal," vol. ix., 1873, which appears to be an abstract of 

 the Proceedings of the Royal Society, 1873, No. 141. 



The present example differs slightly from the figure inasmuch as it 

 has its back lens turned round the other way, so that its flat side is next 

 the object. 



These lenses did not prove successful, and were soon given up. 

 Similar objectives were made by Powell and Lealand, but they dis- 

 continued their production after they had brought out their " new 

 formula " which had separate fronts for dry and water immersion work. 



This Ross lens is a ^ by measurement ; its aperture is 0"81 dry, and 

 0*83 wet, so that it would be quite incapable of resolving Amphipleura 

 pellucida. 



The figure attached is not to scale, but is given to show the form of 

 the lenses. 



Howland's Instrument for Centring, Marking, and Testing 

 Lenses.* — C. W. Howland has patented this instrument, which is in- 

 tended to centre, or decentre lenses, as desired. It also axis-marks 

 cylindrical lenses, so that they can be cut to any axis without adjusting 

 the lens-cutter. The instrument also shows the strength of prismatic 

 lenses, and it tests finished prescriptions, indicating whether they have 

 been filled correctly. 



The frame of the instrument comprises a stand with a bed, on which 

 are three brackets (fig. 30). At one end is a fixed bracket with a sight 

 tube, and a blinker for the eye not in use. The standard at the other end 

 carries a protractor, in which is a target revolvable by turning a rod at the 

 right-hand side of the bed, looking towards the target. The same serves 

 as a push-and-pull rod to adjust the position of the target along the bed, 

 according to the focus of the lens. At the left side is a link, pivoted to 



* Optical Instrument Monthly (New York) i. No. 3 (Aug. 1905) pp. 24-6 (1 pi. 

 and 8 figs.). 



