Oct. 27, 18S1] 



NATURE 



619 



required in any individual case. Tuo circular frames each 2\ 

 inches in diameter, and with teeth cut in their ed^jes, are mounted, 

 so that the teeth gear into each other, and they can rotate freely, 

 but in opposite directions. In the centre of each frame is 

 mounted a prism of 18°; one of the frames is graduated, and 

 when the graduation is at 0° the axes of the prisms ai'e parallel, 

 so that parallel pencils of rays falling on both are deviated both 

 iu the same direction, and still parallel. Thus when the pair of 

 prisms are arranged liorizontally in front of a pair of eyes, an 

 object looked at appears displaced up or down, but there is no 

 lateral deviation on either. If the frames bi rotated 90° in one 

 direction, the prisms both have their bases inwards, or, if in the 

 other direction, both outwards, so that two pencils of rays 

 are deviated to the full power of the prisms. In the in- 

 termediate positions part of the prismatic effect is resolved in 

 a direction at right angles t5 the Inie joining the centres of the 

 frames, and can be neglected as only producing parallel dis- 

 placement of the image, and part is resolved in the direction of 

 this line so as to produce apparent separation or approximation 

 of the images. This amount is read off from the graduation, 

 which is constructed on the following principle: — Suppose a 

 ray of light X A O perpendicular to the plane of the pii>er 

 meets the paper at O. Suppose a prism be introduced at A 



having an angle of deflection 9, the ray of light now falls on 

 the paper at B. If the prism be rotated through angle /3, the 

 ray now falls on the paper at C. Join O B, 6 C, and resolve 

 O C into vertical and horizontal co-ordinates C D, O D. CD 

 being neglected as described, we \vi h to find O D the horizontal 

 component of the deflection. 



Since OB = OC 



. op_ op 



"OB OC 

 OD 



Two other frames are placed in front of the prisms. They 

 contain grooves to hold lenses or combinations of lense.s, and 

 are graduated so that cylindrical lenses can be set at any desired 

 angle. The frames can be separated or brought nearer witli 

 greater accuracy by a wedge, and the distance of the centres of 

 the glasses is marked on the bearing. The whole is carried at 

 the end of a graduated bar which carries a sliding support for an 

 object. This bar is graduated in inches for use in calculation 

 and also in focal lengths of a set of dioptric lenses. A third 

 prism is attached, so that it can be placed between one of the 



frames and tlie object. When it is in position, the rays going 

 through it to the eye appear to come from an object higher 

 than when it is absent. Double vision is produced, and 

 the eyes left free to find their most comfortable position free 

 from any effort to make the two images coalesce. To use the 

 instrument, the spherical and cylindrical elements of the spec- 

 tacle required are first found either by some of the ordinary 

 methods or by the ophthalmometer described in the Annual 

 Volume for 1S80, and the required lenses from the trial case put 

 in the appropriate frames. The third prism is interposed, and 

 an object, such as a vertical line, looked at at reading distance. 

 If the images seen by the two eyes are exactly one above the 

 other, the prismatic adjustment is presumably correct, the third 

 prism is removed, and trial is made whether reading can be 

 carried on for some time without fatigue. If the images are 

 slightly displaced externally, trial is made w hether shifting the 

 cenU'es of the lenses nearer or further off suffices to bring them 

 into position. If so, the distance is noted and sent as a direction 

 to the optician. If the displacement be more than can be cor- 

 rected by this means, the prisms are rotated till the desired effect 

 is produced, and the amoujit of prismatic deviation to be given 

 to the proposed spe'^taclei read off. The third prism is remo\ed, 

 and reading practised as above. 



SCIENCE A AD INDUSTRY "■ 

 /^OXSIDERIXG the high position in literature and science 

 ^-^ of my predecessors in this chair, I feel that I have been 

 bold indeed in accepting the distinguished oflice of President of 

 the Midland Institute during the current year. I shall not 

 attempt 1 1 rival my predece-sor- in those literary or philosophic 

 flights which befitted their powers, but shall confine myself to 

 certain suggestive remarks flowing from personal experience 

 of men and matter, which may prove of some interest to an 

 audience consisting in the main of ])ersons who, like myself, are 

 intent upon combining science with practical aims, but who, 

 unlike myseU, have the best part of their career still before 

 them. 



In venturing to expre-s my views regarding the great question 

 of the day, that of Technical Education, I shall run considerable 

 risk of disappointing some of its most ardent advocates, who 

 may have looked upon me, a foreigner by birth, as a staunch 

 supporter, if not as the living emtiodiment, of that particular 

 form of education that the Polyteohnicum of Germany and other 

 Continental countries imparts to the aspiring engineer and manu- 

 facturer, but which, in my opinion, leaves much to be desired, 

 and is certainly inapplicable to the condition of things which we 

 find in this country. 



The subject of education, and of science edncatioa in par- 

 ticular, is one the practical and national importance of which it 

 would be diflicult to over-estimate. "It is well known that the 

 Continental nations have in some respects stolen a march upon 

 us in providing for the education of the young engineer, the 

 architect, the manufac'urer, and the crafts.nan. Colleges of 

 high and low degree abound where b ith science and practical 

 processes are taught, whereas the teaching of the latter has been 

 1 joked on hitherto amongst us as professional or trade know- 

 ledge to be acquired during lengthy periods of pupillage or 

 apprentice^hip. 



The more ardent advocates of the Continental method of 

 technical education go so far as to think that the irksome system 

 of apprenticeship should give way entirely to technical teaching 

 within the college walls, whereby it is assumed much time could 

 be saved and a better knowledge be imparted to the aspiring 

 engineer or manufacturer. Having had some experience of 

 young men brought up at these technical schools, I am bound to 

 say thrt I have not been favourably impressed with the results 

 produced by that system. The practical know ledge acquired at 

 those establishments is wanting in what may be called the com- 

 mercial element, that is of due regard to cost of production, of 

 which the teacher himself must be comparatively ignorant, 'as 

 otlierw ise we should find him employed at the factory or engi- 

 neering office, instead of in the schoolroom. 



The young polytechnic student is apt to look on the machine 

 or process which he has studied, not as one of many solutions of 

 a practical problem influenced by ever-varying external circum- 

 stances, but as something representing an absolute condition of 

 things almost as completely proved and established as a first 



I Abstract of Address at the opening of the Birmingham Midland Insti- 

 tute, by Dr. C. W. Siemens, F.R.S., president of the Instrtu'.e. 



