762 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the milled head is operated. Being independent of racks and screws, 

 no back-lash can ocenr. The range of motion is about l£ in. 



Fig. 167. 



A Lens Pseudoscope.* — The Wheatstone pseudoscope is composed 

 of two totally reflecting prisms arranged with their edges perpendicular 

 to the plane of vision. ll.Bowden has arranged a pseudoscope in which 

 he employs two pairs of identical convex lenses. He contrives a bar- 

 frame like a capital H, the four lenses being set in sliding mounts on 

 the outer lines. A handle in the middle of the cross-bar and perpen- 

 dicular to the frame makes a good holder. One of the observer's eyes 

 looks at an object through one pair of the lenses, and his other eye 

 views it through the other pair. The planes of the lenses are so dis- 

 posed that their foci coincide, and thus superimposed images are presented 

 to the observer. It was found that the illusions produced were very 

 complete, and had a superiority over the Wheatstone pseudoscope ; but 

 the images presented by it are inverted as well as transposed from right 

 to left. 



(2) Eye-pieces and Objectives. 



Graphic Representation of the Correction Distance of an Objec- 

 tive.f — H. Schmidt shows how, in the absence of the appropriate instru- 

 ment, an idea of the astigmatism of a lens may be obtained. It is 

 necessary to calculate, from the formulae, the horizontal and vertical 

 foci of oblique rays, and then to plot them to scale on paper. A curve 

 should then be drawn free-hand through each set of foci. If these 

 curves coincide the astigmatism will be nil ; it will also vanish at points 

 where they intersect : the amount of divergence on any ray will indicate 

 the astigmatic difference for that ray. It may sometimes be desirable 

 to plot on a magnified scale when the curves show close approximation. 



The Injurious Effect of Cement upon Objectives.} — Gr. Eberhard 

 found that the zonal errors of certain telescope objectives markedly 



* Trav. Sci.Uuiv. Rennes, i. (1902) pp. 157-103 (2 figs.). 



+ Central-Zeit. f. Opt. und Mech., xxiv. (1903) pp. 73-5 (3 figs.). 



% Zeit. f. Instr., xxiii. (1903) pp. 274-7 (2 figs.). 



