314 SUMMAET OP OUBEENT RESEARCHES RELATING TO 



requiring plus and minus lenses of both kinds, ranging in focal 

 distance from six incites in extreme to seventy or one hundred in mild 

 cases, and skill and experience are necessary to correct astigmatism. 

 How then can the ordinary microscopist bring order out of confusion 

 by experimenting with three eye-glasses ? 



It would improve matters if the astigmatic observer were to 

 have the formula for a lens neutralizing his asymmetry (for instance, 

 thus : — 36 cyl. axis 180°) sent to an optician, and have made a small 

 lens, with a setting of thin brass, so constructed as to slip on the top 

 of the eye-piece, over the eye-lens and as close to it as possible. The 

 slight details of convenient construction readily suggest themselves to 

 the optician. 



If the microscopist uses the eye-piece of one maker, the accessory, 

 for such it is, fits each and all of them as they are brought into use, 

 and when not needed it may be easily removed, leaving the perfect 

 eye-piece ready for use under the normal eye. 



Many, in fact most, astigmatic persons have a different degree of 

 defect in each eye, and therefore a better plan would be to have 

 suitable cylindrical lenses put into spectacle frames, and worn only 

 while using the Microscope. These can be placed near the eyes, 

 the axis of each is firmly held in its proper relation to the effective 

 medium and each eye has before it the exact correction of that eye's 

 asymmetry. To be sure, this requires the aid of a skilled specialist, 

 but once done, there is no further trouble or anxiety — no examination 

 with test-lines in order to be sure that the glasses are in the best 

 position for work. 



Secondary distortion because of being a little distance from the 

 eye-lens of the instrument is not troublesome, nor worth considering 

 as against convenience, comfort, and the ability to instantly change 

 eyes when working — an important desideratum. 



The author also thinks that many of the disputes between 

 microscopists as to the markings of test and other objects, notably 

 those having lines meeting or crossing at various angles, are possibly 

 due to the fact that they are not seen through optically similar eyes, 

 one being practically free from astigmatism and the other having it 

 developed to a much greater degree, thus making it utterly impossible 

 for the observers to see alike. 



Malassez's Camera Lucida.* — M. L. Malassez discusses cameraB 

 lucidae in general, and describes a modification which he has designed 

 to avoid the inconveniences attendant upon the existing forms, and 

 particularly the necessity of placing the Microscope vertical and 

 drawing on an inclined plane in order to insure the correspondence 

 of drawing and object. It is much preferable to be able to have the 

 Microscope in an inclined position and the paper horizontal. 



If a Doyere and Milne-Edwards or Nachet camera is placed on a 

 Microscope inclined 15°-18°, so that the image is thrown behind the 

 Microscope, it will be found that it is partly projected on the base. 



* Laborat. d'Histol. du College de France. Travaux de 1884 (1885) pp. 166-79 

 (1 fig.)- 



