276 Br Hartridge, Colourimeter Design 



colourimeter, since it is due to the particular method of illumina- 

 tion described above. 



The eyepiece used in the du Bosq type of colourimeter consists 

 of a Eamsden lens system, at the upper focal plane of which has 

 been placed a diaphragm pierced with a small aperture. This has 

 the effect of limiting the rays reaching the eye to those which 

 have passed as approximately parallel bundles up the limbs of the 

 instrument. To be effective the aperture has to be small, and this 

 has the disadvantage of making the intensity of illumination of 

 the fields somewhat low. When this type of eyepiece is in use it 

 is found that the eye has to be inconveniently close to the aperture 

 in order that the whole field shall be seen at one and the same 

 time. This is due to the fact that the diaphragm is a considerable 

 distance below the effective pupil of the eye, even when the eye 

 has been placed as close as possible, and as a result some of the 

 rays which spread out from the diaphragm may not enter the pupil. 

 The difficulty is in fact similar to that met with in high power 

 microscopic eyepieces of the Huygenian type. To avoid this diffi- 

 culty a more elaborate type of eyepiece has been devised, in which 

 an erecting lens system has been placed above the Ramsden ocular 

 and its diaphragm (8). This causes a sharp image to be seen on 

 looking down the eyepiece, and at the same time the image of the 

 small aperture is formed at a considerable distance above the top 

 lens, so that the eye does not have to be placed inconveniently 

 close to the eyepiece in order to obtain a full view of the field. 

 These improvements are obtained, however, at a certain sacrifice 

 of definition, which is unimportant in the usual types of colouri- 

 meter in which the fields are of simple design, but is of relatively 

 greater importance if the more detailed type of field be used which 

 has been described above. It will have been observed that in the 

 colourimeter which I have described above the illuminating beams 

 are formed by the special method of illumination employed. Under 

 which circumstances it is found that the Ramsden disc of the ocular 

 contains the overlapping focussed images of the restricting aper- 

 tures of the lenses Tl and T2, which when the instrument is in 

 correct adjustment exactly overlay one another. It is therefore 

 unnecessary that the eyepiece should contain any diaphragm to 

 restrict the beams, and therefore the difficulties introduced by 

 such a diaphragm are not met with. The eyepiece itself should 

 be achromatic and should slide in a tight-fitting jacket so that the 

 observer may set it at the best focus. It should magnify about 3 

 diameters. 



The angle at which the comparison field lies will be seen to be 

 45 degrees. But since it is enclosed between two pieces of glass, 

 the apparent angle to the eye is reduced in the ratio of the refi'ac- 

 tive indices of glass and air. The apparent angle would therefore 



