| RefNo | HSF/2/1/98 |
| Previous numbers | 32.8.82 and 32.8.82A-C |
| Level | Item |
| Title | Letter from J [John] Herschel, Bangalore [Bengaluru], to 'Dear Pater' [Sir John Frederick William Herschel] |
| Creator | Herschel; John (1837-1921); British army officer and astronomer |
| Date | 10-16 August 1869 |
| Description | He received his father's letter of 14 July that day and apparently his own letter with the sketch had not yet reached him. The suggestion to observe the Sun on a perforated screen and view it through coloured glasses is welcome, but John has reasons for thinking it would not work. He retracted his own claim to Mr [William] Huggins about absorbing media. John lays out his reasoning to his father, which applies to any scheme seeking to dispense with dispersion. He describes the light gathered by an achromatic object glass within a tube, as originating with the Sun or the column of atmosphere in its direction. Indirect sources of light should be minimal and rendered immaterial by the system of internal diaphragms which John employs. The question becomes one of the intensities of prominence light versus diffuse light from the same area of the Sun. He believes that he has evidence to establish that undispersed sky light coming from near the Sun is much stronger. He gives a numbered series of facts for his father to judge what the brightness of the prominence really is, and he discusses these, concluding that the diffuse solar light which forms the background to the prominences must be sifted. He explains the effect of the light from the solar limb on the slit, as contact with the edge of the slit results in vivid flashing. He thinks that the prism proves 'that the bordering air has a very much higher intensity of illumination than the prominences' and if this is so, any diminution of the intense light which also diminishes the prominence light would be ineffective. John discusses the possibility of an effective absorbent, but also its disadvantages. and what can be said in favour of dispersion. The instrument which he proposed to Alex [Alexander Stewart Herschel] requires only two prisms with two surfaces of great perfection. He thinks that such an instrument would be possible, with the quality of silvering then available, giving undreamt of power of dispersion and selection. Only the improvement of the object glass by not using achromaticity remains. He returns to his father's proposal, explaining its practical difficulties. He thinks it would not allow him to see prominences any better than now, and he has seen so many he tires of their beauty. He describes them with a small sketch. [From 16 August] He has received Mr [George Gabriel] Stokes's letter, copied by Bella [Isabella Herschel] and he pleads guilty to abuse of the word 'continuous' in relation to faculae. He intentionally used 'intensified solar spectrum'. He considers the state of the atmosphere and favourable observing conditions, including invisible moisture and agitation of the air, theorising that damp air absorbs hear rays, especially hydrogen rays, and his estimate of this may have been erroneous therefore. He thinks that the thickness of glass which he uses gives more diffusion and he sketches his prisms, one of which was damaged and had to be removed. With regard to Stokes's comment on the absence of a luminous or non-luminous structure below the chromosphere in John's sketch, John does not believe that any such thing exists, saying he has never seen anything to confirm this and thinks it a mistake. He also withholds belief in 'the arborescent form of the F line', of which he gives a sketch. He does not think that other observers have put their slit on a tangent with the limb and suspects that the F image and C image are not identical, noting the outcome of possible future observations. |
| Extent | 16p. |
| Format | Manuscript |
| PhysicalDescription | Ink on paper |
| AccessStatus | Open |
Fellows associated with this archive
| Code | PersonName | Dates |
| NA7955 | Herschel; John (1837 - 1921) | 1837 - 1921 |