Miscellaneous. 351 



On Object-slides of Canary Glass. By Professor Erxst Brvcke. 



The light of a clear blue sky is well known to be very unfavourable 

 for microscopic investigations. It is evident that it is not only the 

 small quantity of the reflected light, but also its composition, that 

 produces the injurious effect. The continental microscopes are 

 generally adapted for the white or slightly yellowish light which is 

 reflected from clouds, and the English ones for the yellowish-red 

 light of the gas-flame partially neutralized by cobalt glass. They 

 are never arranged with special reference to the blue light of the sky, 

 because in those countries where the microscope is principally used, 

 a completely unclouded sky is not the rule, but the exception. 



We also know that the contemplation of any colour which has a 

 certain degree of intensity, and which is difi"used over the whole field of 

 vision, is wearying, and in time injurious, to the eye. Tliis applies 

 not only to red and yellow, but also, to a less extent, to blue and 

 green. For this reason green spectacles have gone out of use, and 

 the blue ones are always selected of a weak colour. 



Lastly, the troublesome and injurious nature of the blue light of 

 the sky for microscopists may also be attributed to a third cause. 

 The ordinary pictures of our microscopic objects are shadow-pictures, 

 which fall upon our retina. Their unity and distinctness must con- 

 sequently be destroyed when light is emitted by the objects them- 

 selves. Oberhauser, therefore, furnishes his microscopes with a paste- 

 board screen, which is used when the low powers are employed with 

 transmitted light, to keep oflF the direct light, in order that it may 

 not be reflected from the object and thus reach the field of vision. 



Now we know, from the investigations of Stokes and Helmholtz, 

 that vegetable and animal tissues are not free from (true) internal 

 dispersion ; and although this is so small that it is not observed at 

 all in the ordinary mixed sun-light, yet it is by no means impossible 

 that it might sometimes have an injurious action upon the microscopic 

 picture, when the rays of great refrangibility have acquired an un- 

 usual preponderance in the light falling on the object. 



All these various circumstances indicate that in the blue light of 

 the sky we should endeavour to weaken the strongly refractive rays 

 in comparison with the less refractive. This may be eff'ected by the 

 insertion of a medium which exerts a strong absorption upon the 

 violet end of the spectrum ; but amongst such media, those which 

 do not completely destroy the absorbed light, but, instead of it, emit 

 rays of greater length of vibration, should be selected. One of the 

 best of these media is canary-glass, and this must be particularly 

 applicable to the purpose, as, according to the description of its 

 optical properties given by Stokes, it will fulfil the above requisites, 

 and it may be procured cheaply and without difficulty. 



Experiments made with it completely fulfil these expectations. Ob- 

 ject-slides of canary-glass considerably improve the blue light of the 

 sky ; and even when we have the hght reflected from white clouds, 

 in certain cases, .the conditions of which are not yet ascertained, it is 



