H. ()N8L0W 211 



wheel, notwithstJiiKiing its inrchjinirnl disjulvanUi^en. h«.H imich in itj< 

 favour, hut was Htially alMitxItiiifd tor a connnrn'ial instruiiHiit railed 

 the " Tint-oinetvr." TUv n-iuson ftir this srK'ctinu was that with th«' 

 colour-wheel tht»n' is ua i\\vihiH\ n( di'tiiiin^ nr rfconlin^ thr (olours «if 

 the (lises employed: it is thoreforr iui|H»ssihlr for any fulun- nhs.rvrr 

 to n»produe«» these discs, and eonsefpiently the colours of any readings 

 taken with them. The " Tintonu'trr " however supplies a unit, whieh 

 though arhitnu'v. is recoverahle and sjitisties the other essentials of a 

 standanl. Also, it is placed on the market at a UKKleratc^ price. The 

 scale consistnS of a series of coIouhmI gbtsses, carefully <lyed and stan- 

 danlistnl by comparison with the ^busses of other scales, so that sev<'ral 

 opjHirt unities for ern)r are introducted. For this rcjuson it must be 

 mlmittinl that the eolniir-wlK-el would he prefenvhle, since with it. there 

 is hut one judi^ment to introduce I'rror. Unfoit unately. !i(» series of 

 sU'imlard colour discs can he pnK'ured. They should be based on some 

 physical constant such jus wave length, so that they could be che-cked 

 eiisily in avse (»f fading. 



It may be added that sometimes the colour-wheel can be used with 

 advantage cond)ined with some simi)le optical arrangement such as is 

 provideti by the "Tintometer," to secure conditions of eipuil illumination. 



11. The "Tintometer'." 



The "Tintometer" (Fig. 1) consists essentially of a rectangular tube 

 B, slightly tapered and about 10 inches long. At the narrow end there 

 is an eye-piece A : at the other end there are two apertures which 

 admit light. The tube is mounted on a base to which it is inclined at 

 an angle of about 45''. Just above the apertures there are two rows of 

 grooved slots G, which receive the graded standard slips of coloured 

 glass F, for intercepting the beams of light before they reach the eye. 



The apparatus is used tis follows: a piece of mirror is put imme- 

 diately under the two apertures and the instrument placed in diffused 

 daylight, preferably from a north window. The instrument is now 

 moved until both fields of view are equally illuminated ; all objects such 

 as window-sjishes, trees, etc., being ayoided. The unknown coloured 

 object is then phiced under one aperture, and the specially preparefl 

 white background C, made of firmly compressed plaster of Paris, under 



^ Further details for the use of this instrument for other purposes are to be found in 

 Mecuiirement of Light and Colour Setu<ntiom (George Gill and Sons) and Light and Colour 

 Theories (E. & F. N. Spon, Ltd.) by J. W. Lovibond. the inventor. 



15—2 



