X. SPECIAL COLLECTIONS. 961 



may be inferred the lengthening of certain muscles by definite weights at 

 definite degrees of fatigue. One edge of the board is designed for the experi- 

 ment with the left, the other for the experiment with the right, arm. 



e. PHYSIOLOGICAL OPTICS. 



3964. Double Spectacles, for the determination of refrac- 

 tion (Handbuch der Augenheilkunde von Saemisch und Graefe, 

 III., p. 50). Dr. Sncllen, Utrecht. 



This is an opera-glass in the form of a pair of spectacles, consisting of a pair 

 of negative glasses of one inch focal distance, and in front of these a pair of 

 positive glasses of two inches focal distance. On changing the distance of the 

 positive and negative glasses a hand is made to move on a disc furnished with 

 a scale, which indicates the refraction of the system corresponding with the 

 different degrees of sliding. To be had from R. Jung, optician, Heidelberg. 



795. Fhakometer (Snellcn), for the determination of the 

 power of lenses (by the method of placing object and image at 

 equal distances from the lens). Dr. Snellcn, Utrecht. 



The object (points of light on ground glass) and the screen upon winch 

 the image is received are moved in a perfectly similar manner, but in opposite 

 directions, each by a sliding steel spring. 



For the determination of weak lenses, two auxiliary lenses, No. 2*75, are 

 placed one on each side of the lens examined (at a distance of 24-33 mm. 

 measured from centre to centre). 



The screen which receives the image moves alongside a scale, marked at 

 each double focal distance of the system of the three lenses obtained by 

 calculation for the lenses commonly used in ophthalmological practice. 

 Within ^ " dioptric " one can with sufficient accuracy estimate how much the 

 lens examined differs from the powers marked on the scale. 



The image having constantly the same magnitude, precise adjustment is 

 easy. The screen carries a diagram of the image. If the irnnge of the 

 points of light cover this diagram, the centre of the glass will coincide with 

 the diameter of the instrument. Not only the focus, but also the centre can 

 be determined directly. 



The scale may be controlled at any time by determining the strong glasses 

 directly, the auxiliary lenses being removed. Then the double-focal distance 

 is to be taken from the corresponding principal plane to the point where the 

 image is formed. 



The instrument as yet is only adapted for symmetrical (bi-convex) lenses. In 

 order to determine plano-convex or periscopic glasses, it will be best to place 

 two glasses of equal power one against the other, so as to obtain a symmetrical 

 form. According to the calculated principal planes of this system, a scale 

 has to be computed. 



796. Lens of Stokes, with constant axis (Sncllen) ; con- 

 sisting of a negative and a positive cylindrical lens rotating 

 equally, but in opposite directions. (Graefe's Arcliiv fiir Ophthal- 

 mologie, 1873, XIX. 1, p. 70.) Dr. Sncllen, Utrecht. 



Two cylindrical lenses (C T \j- and C +-^) centred one before the other can 

 be made to rotate equally, but in opposite directions, about an axis perpen- 

 dicular to the plane of the glasses, by means of twa sliding steel springs. 

 The principal meridians of the system here remain in the same direction. 

 40075. 3 P 



