[Responding to WT's 1826-2-7], JH thanks WT for materials sent, makes suggestions regarding WT's upcoming trip to Geneva, and asks WT to deliver a book to [Alfred] Gautier.

[Responding to WT's 1826-3-24], JH concurs with P. S. Laplace's view about the advantage of astronomical observation from high elevations, e.g., from mountains. Comments on telescope prices, a paper by WT, Florence, and JH's plan to visit the Continent.

Thanks WT for bringing to JH Josef Fraunhofer's publications and Franz Gruithuisen's plates. Recounts JH's geologizing in France and JH's efforts to measure solar radiation.

Thanks WT for optical equipment sent. Notes death of Augustin Fresnel. Has purchased a telescope from James South.

Responds to WT's 1827-10-29. Sending JH's Light, which uses some of WT's publications.

Asks about a servant JH is considering hiring to accompany JH on JH's trip to the Continent.

Signing certificates for election to fellowship in the R.S.L. at this time [JH having been nominated for President] would be very awkward for JH, so he will not sign for WT.

Sending WT some seeds. Had hoped to send some bulbs.

Is sending WT some botanical specimens through Andrew Smith, whose scientific abilities JH praises. Asks WT to assist Smith in England. Encourages WT's work in mathematics and in 'photology.' Reports on JH's astronomical observations and theorizes about sunspots.

Thanks for specimens of [light] 'sensitive paper.' Praises it. JH has handed over all his specimens of photography to R.S.L. Recommends a paper by the chemist Henri Regnault.

Thanks for photographic specimens WT sent. Reports on JH's recent experiments, including some using lenses, in photography; comments on WT's experiments and on the process of patenting.

Says that for the next four to five months will be in residence at 10 Hanover Crescent, Regent's Park. Offers various botanical specimens to WT, including a Satyrium named after JH. Mentions Eta Argus. Believes the 'Southern Sky has been pretty well rummaged.'

[Responding to WT's 1839-1-25], JH reports that he cannot come to London to see WT's 'curious process of fixing the image formed by a Camera obscura.' Invites WT to come to Slough.

[Responding to WT's 1839-1-29], urges WT to find some way of giving at least slightly different papers to R.S.L. and Athenaeum. Comments on process of fixing image, referring to a trial of his own.

Considerable comment on procedure for submitting paper to R.S.L. Urges WT to mention JH's method of washing with hyposulfite. Comments on some photographic experiments of WT.

JH welcomes WT's mentioning JH's method of fixing with hyposulfite. JH has put some of JH's images on display at R.S.L. Mentions JH's recent experiments relating to photography. Comments regarding a paper submitted to R.S.L. by WT and on some experiments of WT.

JH will as WT requests stop mentioning JH's hyposulfite fixing process and await publication of WT's process. Reports in detail on some of JH's experiments relating to photography and to spectrum analysis. [In a P.S. dated 1839-2-13,] JH states: 'I have discovered your secret of fixing [or its] equivalent.'

Invites WT to mention JH's hyposulfite process to J. B. Biot. Recounts JH's recent experiments.

Sent WT's specimens to J. B. Biot and F. J. D. Arago in Paris. Developed paper sensitive to heat rays of sun 'beyond the spectrum.' Diagram of three spectra: 'luminous, chemical, and thermic.'

Thanks for specimen of light sensitive paper. Provides hint on the chemical process that Louis Daguerre may have used in his photographic work. Sends specimens of JH's own work.