It says in the Gemorah in Chagigah Daf 16a that if one looks at a rainbow, a Nasi, or Kohanim, it will weaken his eyesight.
How does this Gemorah apply now a days (if it does at all) in regards to looking at a Rav, Rebbe, Rosh Yeshiva, Teacher, Gadol HaDor, etc. Anyone that would assume a role of "leader" in Klal Yisrael (whether it be of a Kehillah, Chassidus, Yeshiva, etc.)
Yehoshua, Yerushalayim, Eretz Yisrael
Dear Yehoshua,
I think that this Gemara is more likely to apply to a non-Jewish king then to a Rosh Yeshivah. The Gemara's source for this Din is the Pasuk: v'Nasatah m'Hodcha Alav. This Pasuk is referring to Moshe transferring power to Yehoshua. When Moshe appoints the 70 Zekeinim to form a Sanhedrin the language that is used is not "Hod" but "Ruach" ("v'Atzalti Min haRuach Asher Aleicha" - baMidbar 11:17).
The word "Hod" is very much associated with rulership. The phrase "Hod Malchus" comes up a couple of times in Tanach (Daniel 11:21 and Divrei haYamim A, 29:25). And, of course, the phrase "Hod v'Hadar" comes up many times and refers to Hashems mastery of the world. In Zecharia (6:13) it says: "v'Hu Yisa Hod v'Yashav u'Mashal". Rashi there on the words "Yisa Hod" says "Hod Nesius" - reminiscent of the wording of our Gemara: Nasi.
In any case, the author of Shemiras haGuf v'haNefesh (Rav Yosef Learner) says (chapter 234, note 3) at the end of his discussion of this Din that this Din is not relevant today because it doesn't say "Talmid Chacham" but rather specifically "Nasi".
b'Yedidus,
Yonasan Sigler
This is not a Psak Halachah