Comments on several experiments with spectra passing through glass films.

Asks JH about Dr. Balfour's account in Asiatic Researches about lunar effects on humans in India.

Comments on use of muriate of lime on plants. Also on existence of unexplained bands on film and on presence of red light beyond normal spectrum with light originating from certain sources.

Questions and comments regarding improvements in microscopy.

[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.

Comments on Josef Fraunhofer's work on double stars, on quality and color of light from different sources, refraction of different kinds of light, and solution of a problem in conic sections. Is going to Geneva and Modena soon.

Describes observations made as he travelled and people visited on journey from London to Ancona.

[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.

Talks about Josef Fraunhofer's failing health and his making of flint glass.

Is sending on two items directed to JH through WT.

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.

Describes prismatic spectrum of flame of cyanogen and offers to repeat experiment with Michael Faraday for JH's benefit.

Asks JH to clarify what goods are his from a joint order with WT from a German optical firm.

Queries and comments about telescopes and microscopes; some thoughts about a diffraction grating.

Renders an accounting of expenses for optical goods received from Germany.

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

Writes in favour of a London University professorship for Thomas Key. Refers to some optical experiments of Michael Faraday, and extends solution of conic section problem first raised in WT's letter of 1826-2-27.

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.